Power Crystal

The costliest part of this Humongous Mecha were the five billion carats of magic crystals.

"The Jewel of Charta! I'm invincible!"

—-- Blackfire, Teen Titans, "Betrothed"

Diamonds are a Magical Girl's best friend. They are also friends to Mons, Doomsday Devices, space rangers, magical knights, Humongous Mecha, and regular old Robots. So if you want something to look highly advanced or incredibly magically powerful, just slap a Mineral MacGuffin on it; just like a normal MacGuffin, it doesn't even have to do anything to be neat!

You see while humanity has been fine for gold and silver to coast by on their prettiness to be worth something, in our fiction we seem intent on making jewellery work for their value and turn them into a universal battery so popular for energy storage it makes the AA look shameful.

In a magical setting, they'll often be called "The Eye of..." and Power Glows will be in full effect. In science fiction settings, they tend to be welcome with the Sufficiently Advanced Alien races. They also can appear in less advanced setting when doing the job of "focusing crystal", not directly making the energy themselves but somehow being able to turn any other energy source into Stuff Blowing Up by having lights bounce around off the internal angles. Your Mad Scientist or Evil Sorcerer will probably be using such a crystal to power his Death Ray.

In a Video Game, these gems are usually the Power Sources and weak points of the robots containing them. They also dim or crack when under a heavy battering, or sometimes even blink like a half-broken LCD display. Robots who are killed will frequently have their crystals dim and go opaque.

Power Crystals may be faceted or completely round and smooth. Depends on the design aesthetic.

Choice spots to bling out your power armor are:

Forehead, as a "Third Eye"
Shoulders, as a military epaulet
Center of the chest (rarely, over the heart), as with "Iron Man"
Below the navel, or as a "belt buckle"
The back of the hand, as with "Lensman"
The palm of the hand, as with Iron Man's Repulsor Rays
On the knees (but for some reason, rarely the top of the feet)

These may not always exactly be crystals or jewels. Oftentimes lenses and emitters also perform this duty, by virtue of their placement.

If extemely powerful, a crystal may be an Amulet of Concentrated Awesome or an Artifact of Doom.

Compare Crystal Spires and Togas, Gem-Encrusted, Green Rocks, All-Natural Gem Polish, Body to Jewel, Crystal Prison, Silver Has Mystic Powers. Supertrope to Data Crystal.

This item is available in the Trope Co catalog.

Examples of Power Crystal include:

Anime and Manga

Sailor Moon and Sailor Chibi-Moon's Power source and transformation devices are the aptly named Silver Crystal. They carry their respective versions in a brooch worn on the bow on their chests. The same crystal is also carried on Sailor Moon's wand during the first season after it's obained and the other girls get a crystal they transform with in the manga. Each senshi is said to have one known as a Sailor Crystal (apparently the the Sailor Starlights carry their Sailor Crystals in their brooches acorrding to the Material's Collection).
Vision of Escaflowne, as the page image shows, the Escaflowne has crystals that seem to serve no function but decoration, and a chest-mounted crystal that's basically the engine—this is where the pilot places a dragon's heart in order to fuel it.
- We actually see the shoulder crystals firing laser beams in one of Dornkirk's visions of a possible future where Van and Hitome 'rally the dragon' to defeat Dornkirk.
Having more in common with Super Robot than its Magical Girl Warrior siblings, Magical Girl Lyrical Nanoha puts a gem on every piece of technology there is; it usually displays what function the machine is using at the time. (Subaru even has the gems on top of her feet.)
The titular Otomes in Mai-Otome use gemstones as their Transformation Trinkets, worn as earrings (and standard rings for their Masters).
Great Mazinger has a blue crystal as a "belt buckle".
And several Mazinger Z Robeasts had Power Crystals in different body areas. An example is Brutus M3, who had two huge diamonds set on its chest worked like a Phlebotinum Battery.
The Angels in Neon Genesis Evangelion have glowing crystal cores which cause the creature to shut down or explode as if their body was made of gelignite when damaged, depending on the needs of the plot. Evas more sensibly hide theirs under several tons of armour.
RahXephon
GaoGaiGar and the other entries in the Brave series frequently have this.
The Tohsaka family specializes in a school of magic devoted to storing magical energy, typically in jewels. Rin does so by sprinkling her blood on her jewels every night, and uses them in battle by throwing them. The downside? Using the jewels generally destroys them, and those jewels are really expensive, so the Tohsaka family always has money problems.
Jetfire in Transformers Armada has "Lensman" hand crystals, but has never been seen using them.
Energon stars in Transformers: Energon (the sequel to the above series) were used by the Transformers to create energon weapons. However, the energon star would eventually run out of energy and vanish, along with the weapons generated by it. Energon stars (and weapons) were colour-coded by faction: red for Autobots and yellow for Decepticons.
Halfway through Slayers Next, Lina acquires a set of four Demon Blood Talismans from Xelloss, which she can use to amplify her magical power. She wears one on her collar, another on her belt, and the others on her wrists; they glow brightly when she channels their power. In addition, she also sports a pair of fake crystals in her shoulderpads, enchanted to explode when removed by a thief.
Both the Rune Gods (while in robot-like mode) and the Knights' Escudo Weapons in Magic Knight Rayearth have significant, color-coded crystals on them.
Skuld in Oh! My Goddess has two on the backs of her hands, and two on top of her feet.
Witchblade has a crystalline "eye" which is visible even in passive form (as a jewel on her "bracer") and glows red when ~~aroused~~ going to kick some ass. See?
Dragon Ball's Chi-chi has one of these in her helmet, used for lasering anything she doesn't like the look of.
In Naruto, Tsunade has one on her forehead which she can use to store and release chakra in order to heal any injury she receives.
SD Gundam has crystals being used in various forms. Musha Gundam makes much of a 'Gundam Crystal', an artifact which is often split into separate parts and handed down to successive generations of heroes. Some versions of the series also have the Mushas house crystals on their torsos, inside of which are their 'GunSouls'. Upon death these crystals exit the body to await reincarnation.
The various Alternate Universe Gundam continuities love this trope.
- In Gundam Wing, all three Wings and the Epyon have large green "search eye" cameras on their chests.
- In Gundam X, the green gem on the GX's chest was the receiver for the microwave power transmission system that powered up the satellite cannon.
- Most of the GN-powered mobile suits in Gundam 00 have these, usually taking the form of one large crystal in the chest and at least one on each limb. They're supposed to be the storage units of the GN particles that make them go.
In Dragon Warrior (Based on the Dragon Quest games), the Red Stone and Blue Stone hold untold power that can only be unleashed by those chosen to guard them. Most importantly, the Red Stone has the power to set loose an all-powerful dragon, who can only be sealed up again by the Blue Stone. The Blue Stone is given to Abel, and while normally it's attached to his headgear, on one occasion he removes the stone and fixes it to a magic staff. Talia is given the Red Stone, which she wears on a necklace.
The Dragon Gems the Hachiyou have on their bodies in Harukanaru Toki no Naka de might count as this, considering they're the source of their powers and are used for performing attacks (or for giving the Miko a power-up, for that matter).
In Tenchi Muyo!, the gems that can interchangeably go either on the hilt of Tenchi's sword or on Ryoko's wrists.
In the three-episode Team Galactic arc of Pokémon Diamond and Pearl, Cyrus uses a crystal of the Red Chain in the palm of his glove to mind-control the legendary Pokémon Uxie, Mesprit and Azelf.
In Guyver there are the Zoa crystals which are embedded into each Zoalord's forehead that give them their power. Some even have other crystals stuck into other parts of their bodies.
Bellcross in Heroic Age has crystals on his chest, forehead, and palms... and making up his shoulder and hip joints.

Comics

Iron Man has crystals in the palm of his hands as focusers for his repulsors. Also, there's a triangular crystal in his chest.
Raven of Teen Titans has a red crystal "Third Eye".
Brutally spoofed in Atomic Robo. While fighting Dr. Dinosaur, Robo articulates several reasons why a time-traveling dinosaur scientist is flatly impossible. After Robo notes that Dr. Dinosaur would have had nothing but fronds and rocks with which to build a time machine, Dinosaur angrily attempts to Hand Wave it by saying "I had CRYSTALS!"
The 99 superhero powers come from magic stones.
The wizard Ogeode's Prisma-Crystal in Crystar Crystal Warrior. He needed it for pretty much any really powerful spell, and it was also the mechanism for turning normal humans into crystal warriors.
The Infinity Gauntlet from the six-part series of the same name required the empowerment of seven gems representing the powers of soul, time, space, mind, reality, power and ego.
One Mickey Mouse comic (a Scrooge McDuck/Mickey Mouse crossover to boot) dealt with the characters chasing after an alien babe who was stealing diamonds. As it turns out, she was needing them to fuel her spaceship. (There is a small Critical Research Failure moment when Mickey apparently figures out that the diamonds must be fuel, because "they contain carbon".)
The various versions of Marvel's Doctor Spectrum of the Squadron Supreme gets his Green Lantern Ring powers from a Power Prism, a large multicolored gem.
It's been suggested several times in assorted media based on DC Comics characters that Kryptonite is a formidable power source. Generally done in the interest of making things harder for Superman.
- Suggested? It was explicitly used as the power source for Metallo in his original incarnation, though with no anti-Superman malice at the time (all the scientist who made Metallo happened to have on hand when he made him was some Kryptonite)
Count Von Deadly in All Fall Down uses one of these—until he explodes.

Fan Works

Lots of examples in With Strings Attached:
- Paul is basically one big human-shaped power crystal—his hair, teeth, and nails are all magical diamond, and his bones may be as well. And he got that way by unknowingly grabbing onto a Trap Gem, which turned him into a diamond statue.
- John's Kansael is a blue teardrop-shaped diamond.
- The skahs wizards cast some of their magic through diamonds.

Film

Parodied in Napoleon Dynamite; the crotch-frying "time machine" that Uncle Rico ordered online needed you to insert the most vital part: power crystals. Which probably were just pieces of quartz.
In the Superman films, crystals activate the holographic systems in the Fortress of Solitude. They can also remove (and, in rare cases, restore) Kryptonian powers.
In The Dark Crystal, the titular crystal must be repaired by Gelfling hand to "set things right."
In Atlantis: The Lost Empire, the Atlanteans have small crystals with evident healing powers, and a big huge one that's under the city that keeps the ecosystem down there going.
Lightsabers in Star Wars utilize special crystals that are required to make the 'blade' of the saber. The crystals are found in caves on the planet Ilum and then imbued with power using the force though meditation.
- According to Knights of the Old Republic, there are many planets where a potential Knight can get a crystal, and different types of crystal can affect the attributes of the lightsaber.
- IIRC you can use any type of crystal/gemstone (and then some) for a lightsaber, just that the Adegan crystals are the most convenient for the Jedi. The gemstone Tenel Ka used for her saber is actually a silicon-based lifeform.
- When the Jedi fell and the Empire rose, the Emperor had almost all of the known sources of lightsaber crystals destroyed and made crystals illegal to possess and trade. For his own purposes he used a special furnace to create the red lightsaber crystals he and his apprentices used. When Luke Skywalker had to create his replacement lightsaber he found such a furnace in Obi-Wan Kenobi's hut, along with instructions on how to build it.
The glowing sensor crystal at the base of the Ilia-probe's neck in Star Trek: The Motion Picture Spock, upon seeing the giant image of Ilia in V'Ger's memory, says "The sensor must contain some special meaning. I must attempt to mind meld with it." He does, finds out that it's a direct link to V'Ger, and the power of V'Ger's mind knocks him unconscious.
In the original Toy Story, Buzz asks Woody if they still use fossil fuels or if they have discovered crystallic fusion.

Literature

In Marion Zimmer Bradley's Darkover novels the "starstones" (aka "matrix cyrstals") can amplify a person's Psychic Powers to the extent where they can achieve magic-like effects. One character observes that a particularly large and powerful matrix could "pull one of the smaller moons down out of it's orbit".
The time machine in The Time Machine is made mostly of quartz carved into a mildly impossible shape.
In the Inheritance Cycle, gems are capable of storing massive amounts of magic, allowing a mage or dragon rider to store days and weeks worth of magic, so that they have a massive supply at their side to use at once, as opposed to merely whatever is in their bodies. For this reason, dragon riders' swords are set with gems, and may carry extras around, as well. Eragon often stores magical energy in the jeweled Belt of Beloth the Wise, or else in Brom's ring.
According to the Young Jedi Knights Star Wars novels, lightsabers are powered by a simple battery, converter, and "focus crystal." Apparently just clarity and sparkliness is required of said crystals, Jedi can "sense" if it's good or not. (They don't even apparently need to be of any particular mineral.) If they're not sparkly enough, the lightsaber will explode.
- Other Expanded Universe sources expand on the idea. Lightsabers with multiple crystals can have more than one length setting. The crystal may be responsible for the color of the blade, though the colors of the crystal and blade need not match. Not all crystalline materials will work; in one case, Corran Horn constructed a blade with two length settings, but the extended length didn't work when he used a fake diamond.
  - In fact, it shorted out rather spectacularly, causing both Corran and his enemy to be extremely surprised...and causing his enemy to drop the deadman switch he was holding.
Dinotopia's sunstones serve as the power source for any and all Lost Technology found by the protagonists.
David Eddings appears to be fond of this trope... both of his main universes - the Belgariad and Sparhawk settings - features a gemstone with enough power to give GODS a case of the shivers. Fortunately, these artifacts - the Orb of Aldur and the Bhelliom Blue-Rose, respectively - are in the hands of the heroes. The Malloreon, however (a sequel to the Belgariad) also features an equally-powerful but EVIL gemstone, the Sardion.
The mages in Avalon: Web of Magic each have an item of jewelry that "channels" their magic. Also, the main plot of the last six books is finding the Nine Power Crystals of Avalon in order to stop the flood of bad magic.
Examples in the Shannara series include the Diapson Crystals which convert solar energy to power airships and the Elfstones, which have so many uses (sometimes unintentional) as to serve as a veritable MacGuffin for the series.
The Codex Alera series features a variant, in that the Canim bloodstones don't power their magic but protect the wearer from it. They are usually set in the hilts of their weapons or in amulets.
The currency in The Way of Kings is made of crystals that store Stormlight. On top of that, they are luminescent when filled with Stormlight, so many kings use them to illuminate their hallways. A very practical solution. Until an assassin well versed in magic comes for you.

Live Action TV

Ultraman uses this as an indicator as to how long his power supply has left.
Some of the Megazords from Power Rangers.
- The Zeo crystals, which give the team a new set of powers as well.
The Phormo and Synchro combined forms from Superhuman Samurai Syber-Squad.
Star Trek dilithium comes in crystals, and in the original series must remain in that form to work.
- Later Techno Babble describes it as not the actual power source, but as a regulating agent for the Matter / Antimatter reaction.
Land of the Lost had a lot of these, with varying effects from dimensional gateway controls to impromptu grenades.
In both Stargate SG-1 and Stargate Atlantis, virtually all technology from the Sufficiently Advanced Aliens has crystals at its most basic level. Goa'uld have rainbow-colored geometric ones, the Ancients' are clear flat rectangles.
- In both cases the crystals are mainly used for data storage, which is actually something that crystals can be used for in real life through holographic memory.
- Then there's the Zero Point Module, perennial MacGuffin and power source. It looks like an orange, glowing lump of quartz.
An early episode of Angel featured a demon with a jewel in its head which had to be smashed in order to permanently kill it.
Firefly's data storage units use square crystals. Truth in Television, sort of, as above with Stargate SG-1.
Babylon 5 also has datacrystals.
In the Doctor Who Classic story "The Horns of Nimon", putting a special crystal into the ceremonial staff the Nimon give to their spokespeople turns it into an energy weapon that can stun the Nimon.
- The Metebelis Crystals seen in "The Green Death" and "Planet of the Spiders".

Religion, Mythology, & Philosophy

YMMV of course on whether it's this or Real Life, but according to Edgar Cayce and others, Atlantis made heavy use of these, and it was part of what led to their downfall.
Many New Agers and neopagans believe crystals have special properties that enhance certain qualities or aid in healing.
Orgonite. It's not actually a true crystal, but molded resin (eg, plastic) filled with bits of minerals and shiny things. The purpose is to absorb bad "orgones," or negative energy. Whether or not they actually work, some peoples' orgonites are quite beautiful.

Tabletop Games

The Craftworld Eldar of Warhammer 40,000 universally wear "Waystones," a gem worn over the heart which captures the soul of the Eldar upon death to prevent it from being painfully devoured by the Chaos god Slaanesh. These spirit stones allow the Eldar a peculiar kind of necromancy with Wraithguard and Wraithlords, battle constructs controlled by the spirit of a long-dead Eldar in a waystone. The Eldar also pimp out their vehicles and weapons with countless more mundane gems.
- They also occasionally have starships and titans powered by spirit stones. Though, since they already have a light cruiser class called the 'wraithship,' they had to call spirit-powered starships 'ghostships'
Dungeons & Dragons are full of magic jewelry, both standalone and embedded in other items.
- Dungeons & Dragons 3.5 introduced a class over the internet, whose schtick involves embedding crystals into their skin. Precious gemstones specifically.
  - Years before Dungeons & Dragons 3rd Edition came out, the Earthdawn roleplaying game featured not only crystal plate mail and shields, but also living crystal armor (small columns) and so-called "blood pebbles" (hundreds of small magical gems) that can be implanted into a character's skin and grow by feeding on the magic of the character's blood until they provide a natural armor bonus on the whole body (except the head). Because the crystal is alive and part of the character's body, the character can feel and move with it as if the crystal was his skin.
- Many spells involved gems as component or focus (e.g. Gemjump - teleport to prepared gem). Sourcebooks on magic items and materials (like Volo's Guide to All Things Magical) have lots of gem-related entries.
- Psionics has a particular affinity with crystal - most psionic items are made at least partially out of crystal, psions can acquire a "psicrystal" companion, and there are psionic powers that can summon shards of crystal to attack enemies with.
  - Not to mention "power-up" crystals embedded in armor and other items that make them quasi-magical.
Hearthstones in Exalted are magical gems produced via geomancy that provide their user with energy and special abilities when attached to an attuned artifact (weapon, armor, amulet etc.).
GURPS has "powerstones" (which are always made from gems) that are pretty much absolutely required to provide extra energy for casting any powerful spell as the only other alternative is horribly ironic death as the casting rips your life force out of your body.

Theme Parks

Various crystals have appeared in Disney Theme Parks attractions
- The Indiana Jones Adventure has hinted within the queue that Indy became lost on his search for the tourists going after a Gem of Power that provided the energy source to the entire Temple of the Forbidden Eye
- The Sorcerers of the Magic Kingdom game has Hades and his army of Disney villains seeking out pieces of the Crystal of the Magic Kingdom that could allow him control over the park and the Disney worlds within.

Video Games

Pick a Mega Man game. Any game. Especially during/after Mega Man X.
- Special mention goes to Zero's "booblights".
  - Zero's Power Crystal also has his "Z" insignia plus a little relevance. It might be his weakness, as a Flash Back in X4 shows that Sigma couldn't have defeated him if it weren't for Zero convulsing when it was flashing.
- The red one in X's helmet has a couple of functions besides looking neat. If his health is low, it blinks on and off while he's standing still. In X3, the level maps you can access after acquiring the helmet upgrade appear to be projected from the gem like a hologram. In terms of gems that are just there for show, many of X's special armors have ones in various spots. In particular, his Blade Armor from X6 has green gems on his arms, shoulders, chest, waist, and the tops of his leg guards.
Samus has a "Lensman" hand crystal on her non-cannon arm. In Zero Mission, it corresponds to the Power Grip, and in Metroid Prime 3, the Grapple Beam is fired from it.
Some Pokémon have gems encrusted in them for show: Staryu and Starmie, Golduck, Persian, Espeon, Ampharos, Sableye, Vespiquen, etc. Suicune deserves special mention, as he has a big friggin' crystal encrusted in its head. There is also an attack called "Power gem".
- Pokedex entries for Staryu/Starmie claim that their crystal IS actually their eye, which is based on "compound eyes" of some real starfish.
- Appropriately enough, Dialga and Palkia have a diamond and pearl on their bodies, respectively. The events of The Rise Of Darkrai are set off when Palkia's pearl is damaged.
- Emerald of the Manga has one on his forehead.
- The crystal in Deoxys' body is actually its brain. In fact, as long as that is intact, it's still alive, considering its Healing Factor allows it to regenerate in the right conditions.
The latest Nazi plot in Wolfenstein revolves around the so-called Nachtsonne Crystals found near Isenstadt, Germany. Interestingly, the crystals themselves hold no intrinsic power, but are instead used as "lenses" for the Black Sun energy leaking into our universe. The Nazis harvest the energy, storing it in special barrels, which they use to fuel various crystal-based contraptions. Different crystals have different results: some slow down time, while others are used to create Some Kind of Force Field or activate the Thule Portal.
Zone of the Enders. The more of them your Orbital Frame has, the stronger it's likely to be.
The Savior from Devil May Cry 4 had blue gems on his forehead, his chest, and both of his arms. For best results in order to save Nero, Dante has to destroy them all, though only damage to the gem on the chest counts as damage to the boss himself.
The Jumi from Legend of Mana have gems as their hearts. It gives them unlimited life and lots of power, but if it's so much as scratched, never mind destroyed, bad things happen.
In StarCraft, Protoss technology relies on Khaydarin crystals, which can focus, store, and amplify their Psychic Powers. The pylons, the ones you need more of, are basically giant floating chunks of the stuff. The "minerals" that are harvested by all three races are also some kind of crystal.
- Protoss units are also often adorned with power crystals. Starcraft 2 has some units, like the Warp Prism and Void Ray which are built around them.
- This gets amusing in Starcraft 2, when a Protoss crystal is brought on board the Hyperion and studied in its lab. The crystal grows and eventually begins emitting a lot of power, but the lab tech cannot figure out where said power is going. Turns out the crystal had tied itself into the ship's power grid without anybody noticing, and is now helping to regulate the ship's systems and keep them all at optimal performance levels.
Sins of a Solar Empire has Crystal as one of it's three resources used by all of the 3 factions. The Advent are particularly fond of it.
The all-important Spirit Gems that Jeanne D'Arc's heroes wear on their armlets. Each Gem grants a new transformation (each character can wield between three and five, depending on the slots on their armlet) and two of them can be Lost Forever if the player doesn't have sufficient warning.
The chaos emeralds from the Sonic the Hedgehog games are the epitome of this trope. There are seven of them, and with even a single fake chaos emerald one can teleport. A single real one is all that it takes to essentially freeze time in a decently sized area. Two chaos emeralds are capable of opening a portal through time. And when you get all seven, its pretty much the epitome of Super Mode.
- And then there's the Master Emerald, professional MacGuffin and the power source that keeps Angel Island a Floating Continent.
- An example of this meets Attack Animal is Chaos from Sonic Adventure, an (according to Wikipedia) "artificially intelligent, virtually generated life-form" that is powered by "Chaos Emeralds", and wrecks cities and whatnot.
- And in Sonic Adventure 2, there's this thing called the Eclipse Cannon. The more Emeralds it has, the more powerful it is, and even when it doesn't have the full complement, it still takes out a good-sized chunk of the moon.
The Elemental Crystals from various Final Fantasy games. Their exact powers vary from game to game, but they're usually the main Cosmic Keystones of the world, maintaining and controlling the four elements. In some games, their powers can be harnessed and amplified, and in a few, even a single shard of a broken Crystal has enough power in it for a person to transform into a hero.
- They also tend to keep the Sealed Evil in a Can... sealed.
- They're even more central in Fabula Nova Crystallis.
  - Specifically, in Final Fantasy XIII, they are considered bad things, since they mean either Cie'thification (and your soul turns into a palm sized crystal, which can be collected, sold, or destroyed by strangers,) or eternal life at the cost of your consciousness.
Amarr ships in EVE Online primarily use lasers. Fitting different crystals to the laser turrets affects attributes such as optimal range, damage type, and even what color the laser is.
Naaru Magitech in World of Warcraft seems to rely on these, at least judging from the amount of large crystals in the Tempest Keep instances. The Naaru themselves seems like sentient PowerCrystals / Energy Beings. The draenei use a lot of their crystals too.
- Un'goro Crater has PowerCrystals of various colours scattered around the zone, as well as pylons that give various buffs when you bring crystals of correct colour to them. Several other magical crystals appear in the game too. Un'goro's crystal pylons are part of a huge homage to Land of the Lost.
- Kael'thas Sunstrider in Magister's Terrace has a huge demonic crystal stuck in his chest.
- Let's not forget the Jewelcrafting profession, the whole point of which, besides creating wearable stat-enhancing rings, necklaces and trinkets, is the cutting of gems able to enhance the stats of weapons and gear you socket them into.
The Zelda series likes this; magical artifacts like the three spiritual stones in Ocarina of Time are often ornate gemstones with gold decorations or the like, while in the same game the gilver and golden gauntlets have red gemstones set into them, possibly the source of the extra strength they give you. In addition, the Master Sword, in all its appearances, has a yellow gem set in the hilt where the blade meets the crossguard.
Roland from Luminous Arc 2 has a Lensman-style artificial crystal on the back of his hand. It turns witches into his magic-giving harem.
Perhaps the first western example was in Crash Bandicoot 2: Cortex Strikes Back.
Used in KoTOR 1 and 2. Crystals are used to focus lightsaber beams.
Psynergy Stones from Golden Sun.
In Tales of Symphonia, Exspheres are crystals that can be used to power machinery, and also enhance people's fighting abilities if equipped properly. Just sticking it on the skin has a rather different effect, though...
Timonium deposits in Rise of Legends look suspiciously like massive crystal formations
- Fun fact, these crystals are used for EVERY RACE, including the non-technology based Alin. It is stated on the site itself that Timonium has incredible properties, including mystical ones (so presumably, Alin use it to power spells. Doesn't explain why the Hero Unit just use Mana instead.)
In Castlevania: Curse of Darkness, pretty much every Innocent Devil has at least one crystal somewhere.
In Rudra no Hihou, each of the heroes chosen by destiny gets a jade lodged into their forehead. The impact ends up killing one of them and sending him straight to the afterlife.
Halo has the needler a pink gun that shoots pink crystals that don't do much damage and is kind of slow. Wouldn't be your weapon of choice right? Well if the enemy has enough crystals in his body they explode, all of them, at the same time. Also if the victim is carrying plasma grenades expect a big explosion.
Crash Bandicoot. 'nuff said.
In Strife the Evil Empire Religion of Evil runs their technology with crystals. Have fun blowing them up.
Avernum features these a lot, particularly in relation to a): magical portals, b): golems, and c): anything made by the Vahnatai. The bigger ones often go boom nicely.
- From the same developer, Geneforge features crystalline "power coils" as power sources for heavy equipment. These always explode if you screw up when trying to shut them down. Pre-destroyed ones are often surrounded by "flawed crystals" that function as Vendor Trash.
Oblivion had crystals all over Ayelid Ruins, some of which can be taken and used to completely refill the Player Characters magicka or recharge all their magical equipment.
Used as a power source for the ATACs of Vanguard Bandits. They're small enough to fit in your hand, but large enough to power a machine larger than a house.
The Cranassians' red crystals in the Raiden series. The Final Boss is typically a Cores and Turrets Boss powered by large versions of these.
The core of the Weather Control Machine Napishtim in Ys VI, which also summons smaller attack crystals. Also, the elemental crystals of Kefin in Ys V.
In BlazBlue, Ragna the Bloodedge possesses a pair of red crystals on the back of his hands. They act as the activation devices for his Azure Grimoire.
- And Kusanagi has large plates all over her battle suit. For the exact same reason as Ragna. Save that hers are more effective since she is the Inheritor of the Azure.
In Dark Souls, the Primordial Crystal gave Seath the Scaleless true immortality. Also, the most powerful sorceries are crystalline.
In Kingdoms of Amalur, Prismere crystals are used to make all sorts of Magitek for the human races, and the Tuatha use it to make weapons and implant it into their skin. It's seemingly perfectly safe for mortals but drives Fae insane.

Web Animation

Strong Bad of Homestar Runner has a "blue diamond kinda thing" on his forehead which gives him the power to pop caps off of things.

Webcomics

In Gunnerkrigg Court, many of the Robot prototypes have large blue crystals (or possibly just panels of glass) set in their chests. For whatever reason, the production run of the Robots follow a less ornate design and lack the crystals.

Web Original

The cybaspheres from the web fiction serial Dimension Heroes prove to be invaluable trinkets that give the Dimensional Guardians the power they need to defeat the more powerful opponents they face.
There are power gems in the Whateley Universe: Nightgaunt has two on his belt that he uses to shadow-walk; Splendor has several that she bought using daddy's money so she could have superpowers.
Strong Bad has one on his forehead. He can use it to telekinetically pop open a cold one.

Western Animation

From the end of Beast Wars up to the end of Beast Machines, Transformers toys had 'spark crystals', which were translucent plastic gems that had the faction symbol of the character in them. The Transmetal II Blackarachnia toy was special, since it had a spark crystal which could be flipped to display one of the two main faction symbols, reflecting the character switching sides in the cartoon.
You can do a lot of things with Allspark fragments in Transformers Animated. Overload factory equipment, turn regular vehicles into Transformers, bring Transformers back from the dead, power bombs, and putting your Forgotten Superweapon back online is a snap!
In many Transformers continuities, though probably most prominently (first?) in Beast Wars, raw Energon—fuel for Transformers—is depicted as taking a crystalline form.
Storm Hawks features a world where everything is powered by crystals, and there's a crystal for every effect imaginable: hypnosis, explosions, "nitro-boosting" carriers, powering energy swords and boomerangs.
InBarbie and the Diamond Castle, the castle itself is covered with diamonds (hence the name), with a new one added each time a new song is sung. These diamonds also have the power to ward off evil spells.
An episode of Teen Titans had Doctor Light kidnap a girl that could turn into indestructible crystal, to use her as a focusing lens for his superpower machine.
There are 16 different crystals with unique powers in Tenko and the Guardians of the Magic.
The main plot point of "Dressed to Kill," an episode of My Life as a Teenage Robot, was that an extremely powerful crystalline substance was being used in Brit and Tiff's commissioned clothing designs, giving them abilities that they would abuse inevitably.
The Mega Man animated series had shrink rays powered by red jewels, though Dr. Light found a way to alter them to do the polar opposite.
Two of the Doomsday Devices in Inspector Gadget were exactly this, notably the Weather Control Machine in Weather in Tibet, and the satellite-killing laser in The Ruby.
Subverted in Sheep in The Big City, where the new military superweapon has to be powered by the titular sheep: It is a laser cannon with a sheep-shaped slot at the top.
Princess Gwenevere and the Jewel Riders uses them as Transformation Trinkets.
- Robert Mandell, creator of Princess Gwenevere, is the king of this Trope as far as American animation. In his other series, Adventures of the Galaxy Rangers, Starstones and Psychocrystals were dark twins of the Riders' jewels.
In ThunderCats (2011) the Eye of Thundera in the hilt of Lion-O's Sword of Omens is an Amulet of Concentrated Awesome, a gem that allows the sword's chosen wielder to access "Sight Beyond Sight" and turns the weapon itself from a powerless but elaborate dagger into a crackling Sword Beam-shooting longsword when Lion-O utters his By the Power of Greyskull. Big Bad Mumm-Ra claims former ownership, and really wants it back. In "Journey to the Tower of Omens" its revealed that the crystal is actually plundered Imported Alien Phlebotinum the Warstone, which Starfish Aliens used to power a Wave Motion Gun. As of "Legacy," three more crystals exist, once used to create Adaptive Armor, and Mumm-Ra and the Thundercats are in a race to gather them.
In The Smurfs episode "Handy's Window Vision", the Smurfs discover a magic crystal that can transmit images exposed to it unto a bunch of smaller crystals that act as receivers, thus becoming their latest invention, the window vision. Gargamel, however, sees the crystal and uses it as part of his Smurf Smasher machine.

Real Life

Semiconductors, mainly silicon, germanium and various metalloid or nonmetal compounds called III-Vs, are functional crystals whose electronic properties make them extremely useful in a whole lot of roles. This includes use in electronic logic components like diodes and transistors, optical devices like LEDs and lasers, and - in a case of literally being power crystals - solar cells. Every integrated circuit (including your computer microprocessor and memory chips) is essentially a semiconductor crystal, that's been grown, cut, processed in a kind of arcane technological rune magic, and programmed, giving it the ability to create... all this. Although they have the same microstructural shape as diamond, most of these crystals don't actually look that pretty. But they are powerful.
Quartz crystals, a form of silicon dioxide, essentially the stuff of sand. When under pressure it produces an electrical current and it can be set to vibrate to fairly precise frequencies which led to its widespread use in governing electronic clocks, and generating signals for synchronous electronics (basically all modern digital systems).
Tourmaline crystals, like quartz, have the property of "piezoelectricity": when compressed, they develop electrical charges at the ends.
Sapphires and rubies (more rarely, diamonds) are commonly used as bearings in high-quality machinery, including mechanical watch parts. Because of their extreme hardness, they can grind for years and years and not wear down.
Solid state lasers. Confusingly, these aren't the same as semiconductor lasers, instead having a (somewhat) transparent ion-impurity-laced crystal as the lasing medium with an external light-based power source (another laser or a flashlamp) instead of direct electronic power (usually). Indeed, the first laser ever built was a ruby laser, and today sapphire and garnet lasers are indeed pretty common for various applications.
Some New Age and Feng Shui practitioners believe that certain types of crystals have special magical properties. This leads to "crystal healing" with placing various minerals on a person's body, and putting crystals in water which is then treated like holy water (eg, used for rituals). Some people even "charge" their crystals by exposing them to sunlight or moonlight.