Gem Heart

There are plenty of reasons to hunt a dragon. Maybe you want revenge for the village it burnt down. Maybe you're after the fame and fortune that such a kill can bring. Or perhaps you just have a death wish.

For those who live in some universes, however, there's another reason: If you cut open its body, you can find a ruby as big as your fist (or even bigger) nestled in among its squishy bits. This is a Gem Heart.

The creature the Gem Heart is in needn't be a dragon, so long as it's large and dangerous and has a massive precious stone in its body. Sometimes, the stone comes naturally shiny and faceted, other times it doesn't. Often, this gemstone possesses some special magical ability.

In more fantastical universes, this gem actually takes the place (and function) of a major internal organ. This is most often the heart, as the trope name would suggest, but finding gems within the brain isn't uncommon either. In works with a higher degree of biological verisimilitude, these stones are explained away as gizzard stones, used to help the creature break down its food for digestion.

See also Power Crystal. Often a form of Body to Jewel. May overlap with Heart Drive or Immortality Inducer.

Examples of Gem Heart include:

Anime & Manga

Dragons in Vision of Escaflowne have a magical gemstone at their heart that can then be used to power guymelefs.
Organ replacement version in Busou Renkin: Kazuki has a Kakugane in his chest because he blundered into a monster and died, kicking off the plot.

Comic Books

The Superhero Darkhawk is a rare humanoid example, bearing a ruby amulet inside of his chest. At one point, the villain Tombstone tore it out, believing it was valuable. Darkhawk recovered
Requiem Chevalier Vampire: Dragons are killed for the large gemstone that forms naturally in their body.
The MacGuffin in XXXenophile: Heart of Stone was a massive ruby carved from the heart of a monster that induced lust in anyone who touched it with their bare flesh.
Superman's enemy Metallo is called "The Man With The Kryptonite Heart!" thought its actually the power source of his robotic body.

Film

In Sucker Punch, when Babydoll kills the baby dragon, it has rubies in it's throat that she strikes together to spark a fire.

Folklore

Ancient men believed in a stone called a Draconite, which had to be removed from a dragon while it still lived (sites of origin being rumored to include the brain and the heart, this was no simple task). True to the trope, draconites were believed to have magical powers.

Literature

In Juliet E McKenna's Aldabreshin Compass series, natural dragons have a gem that relates to their element for a heart. Unnatural dragons created by wizards will fight very hard against natural dragons because the unnatural seeks to replace the void in its heart with the gem from its natural cousin (and occasionally they will break free of the wizard's control if they can feed on enough gemstones; this is why it is vitally important not to summon a dragon near your treasury). It goes without saying, then, that summoning a dragon is reserved for when things have crossed the Godzilla Threshold.
Greatshells such as the Chasm Fiends in The Stormlight Archive by Brandon Sanderson have gems like raw rubies, emeralds, and sapphires deep beneath their shells. By the time of the story, most of these Gem Hearts are gathered through the slightly unsporting method of waiting until the greatshell is pupating, then cutting through the stone carapace to get to the gem.
Dragons in Eragon have a gemlike organ called an Eldunari. In a twist, though, it has no intrinsic value; it's actually the dragon's emergency Soul Jar.
Two In Michael Moorcock's Elric of Melnibone stories.
- Nanorion stones (gems) are sometimes found as the hearts of demons. They have the magical power to wake the dead or those in a death-like sleep.
- When Corum slays the god Arioch, Arioch's heart turns out to be a huge glowing gemstone.
In Cornelia Funke's Dragonrider, Nettlebrand has a golden casket that acts as his heart. It has no actual value, though.
Discworld trolls have diamond teeth.
Played with in The Hero and The Crown, where the last drop of blood a dragon spills before dying turns into a bloodstone (read: ruby).

Live-Action TV

In the Doctor Who story "Dragonfire", the dragon's treasure turns out to be a large Power Crystal contained within its body. Justified in that the "dragon" is actually a robot built around the crystal to keep it from falling into the wrong hands.

Tabletop Games

In Magic: The Gathering, Karn was created with a Phyrexian heartstone, which worked until he gave up his Planeswalker spark to help mend time rifts on Dominaria. When he did, the oil in the heartstone gradually corrupted him. During the Phyrexian takeover of Mirrodin, the Phyrexians attempted to convert him into their new Father of Machines. Ultimately subverted: his heartstone was replaced with Venser's human heart, restoring his Planeswalker spark and cleansing him of the Phyrexian corruption.
Buzzjewels in Spelljammer. They're swarming insects that resemble large (hand-sized) dragonflies, which graze on "interesting" minerals like gemstones and eventually crystallize some sort of a small gem in their bellies, with a mildly poisonous bite to use if the swarm is attacked.
Vis in Ars Magica, crystallized magic energy, often found coalesced in the magically potent parts of a beast. While not always strictly a gemstone (as the shape it takes is dictated by the storyguide's discretion and the particulars of any individual mage's magic), it often is. A dragon's heart is worth sixty pawns' worth. (To the uninitatied: it's a goddamn fortune.)

Video Games

In Dragon Quest VIII, the party has to go on a Match Maker Quest on the behalf of the Prince of Argonia, who needs to slay an Argon Lizard and harvest their hearts as a sign of his coming of age and the right to marry Princess Medea of Trodain (who, unknown to him, has been transformed into a white mare due to a curse). Unfortunately, this person happens to be Prince Charmles, who is the epitome of entitled bastardry. And to make things worse, after you slay a Great Argon Lizard and do the hard work for him, Prince Charmles buys a larger (and presumably fake) Argon Heart from a shady merchant anyway! Son of a... Luckily, this comes back to bite him in the ass hard, costing him Medea and his undeserved sense of entitlement.
Part of the backstory for Myst V: End of Ages reveals that this is true of the Laki, which appear to be something like otherworldly narwhals. The Laki collect gemstones in their gizzards to help them digest, and (before the cities in their world were destroyed by plague) were hunted almost to extinction in gladiatorial games to harvest them. They survived, but they don't get close enough to for the player to interact with them.
In Legend of Mana, part of the Mana series, there are a race of humans called "Jumi" who all have some sort of precious gem in their chest. If it gets damaged, they could die.
Deoxys from Pokémon Ruby and Sapphire. That gem on its chest is actually its brain.

Real Life

In a less epic vein, oysters, and the pearls they generate (caused by bits of grit covered in mother-of-pearl in order to prevent them from irritating the host bivalve, if you didn't know).
A more tragic example: The value of elephant-tusk ivory and rhinoceros horn (considered an aphrodisiac in some cultures) has led to both creatures being hunted to near-extinction.