Gems

Gems or jewels are prominently featured in several episodes of My Little Pony Friendship is Magic. Unlike actual gemstones, gems in the series are found already cut and polished when extracted from the ground or from rocks. Gems are also eaten by dragons, including Spike.

Appearances
Cut gems are featured throughout the series on jewelry, cutie marks, clothing, and more. Rarity has a particular affinity towards gems: her cutie mark is three lozenge diamonds, and she uses a gem-finding spell in A Dog and Pony Show to get more gems for her gem-studded clothing, the same spell which triggered in The Cutie Mark Chronicles prior to her discovering a rock full of gems. Princess Celestia wears a jewel-studded crown and necklace, and the Elements of Harmony are studded with jewels that match the main ponies' cutie marks. In Sisterhooves Social, Sweetie Belle uses all of Rarity's baby blue sapphires to form the shape of a heart and glues them on a drawing.

Spike's gem-eating diet is first introduced in The Ticket Master where he asks the waiter for rubies. He expresses his love of eating gems in several other episodes, and he's shown eating gems in A Dog and Pony Show, Over a Barrel, Owl's Well That Ends Well, Dragon Quest, Party of One, Secret of My Excess, and Just for Sidekicks. Both the red dragon in Dragonshy and the green dragon in Owl's Well That Ends Well hoard gold and gems in their lairs.

In Just for Sidekicks, it is also shown that ponies can and will accept gems as an alternative form of currency in place of bits. Even one tiny, ruby-colored gem was enough for the Cutie Mark Crusaders to purchase a large, industrial-sized pet dryer. In the same episode, Zecora donates a gem to a Girl Scout-like charity. In Rarity Takes Manehattan, Rarity tips a hotel bellhop with gems and insists that he keep the change.

A Dog and Pony Show
Gems feature prominently in this episode. Rarity runs out of gems for her outfits, and goes searching for them with Spike. She uses a magic spell to locate gems, and Spike digs them out. When they're finished collecting gems, the Diamond Dogs appear and tell Rarity that they "hunt for gems", then kidnap her and force her to find gems for them in their vast underground tunnel system. Spike recruits the other ponies' help, and they eventually find Rarity and leave with several cart-loads of gems, which Spike happily dines on.

The Cutie Mark Chronicles
Rainbow Dash's sonic rainboom splits open a rock full of gems in front of filly Rarity, and she uses them to decorate the costumes for a school production, subsequently earning her her cutie mark.

Secret of My Excess
A special kind of gem which Spike and Rarity identify as a Fire Ruby plays a special role in this episode. Spike shows his heart-shaped Fire Ruby to Twilight and tells her he's been aging it to eat it on his birthday. When Rarity sees it, she dotes on it, and Spike lets her keep it. Rarity makes a necklace out of it for herself and wears it throughout the episode. After Spike turns gigantic because of his greed, he kidnaps Rarity, who doesn't recognize that it's the same Spike. He tries to steal the gem from Rarity, but she holds on to it and describes how generous is the dragon who gave her the gem. Spike has a flashback to the moment he gave Rarity the gem, and shrinks back to his normal size.

After Spike gives Rarity the Fire Ruby, Pinkie Pie tells Spike that Mr. and Mrs. Cake have a "special surprise" for him since it's his birthday. When he enters Sugarcube Corner, the Cakes present him with a blue cupcake studded with sapphires.

Magic Duel
A red diamond-shaped gem is seen on the Alicorn Amulet, a powerful necklace that gave Trixie a lot of magic power. The power of the gem in this episode is used to defeat Twilight in a "Magic Duel." After Twilight finds out Trixie used the Alicorn Amulet to win, she challenges her to another duel using a fake amulet given to her from Zecora, and the help from her friends. After Twilight beats Trixie in the duel, Trixie takes Twilight's fake amulet. Rainbow Dash then takes the Alicorn Amulet and Trixie attempts to attack Rainbow Dash with her magic, but shortly Trixie learns that the amulet that Twilight had was actually a doorstop from Zecora's house.

Just for Sidekicks
Spike's appetite for gems drives the plot for this episode. After he accidentally eats all the gems meant for a jewel cake he was mixing, Fluttershy happens to drop by to ask him if he can pet sit Angel, while she and the rest of the Mane 6 are away at the Crystal Empire, and offers a large green gem as payment. Seeing this as the perfect opportunity to replace the ingredients he ate, Spike offers to look after the pets of the others, in exchange for a gem from each owner. However, as his preoccupation with his future jewel cake takes up most of his thoughts, he doesn't take care of the pets as well as he said he would. This irresponsibility causes him to lose his gems one by one as payment for various mishaps, until he only has half of Fluttershy's gem left. He then, ironically, accidently devours it too when he meant to drop it into his cake batter.

Bats!
Pinkie Pie, afraid of Fluttershy following her transformation into "Flutterbat", burrows into the soil of Sweet Apple Acres. Various gemstones are shown to be scattered beneath the soil's surface.

Nightmare Rarity (story arc)
Spike carries another fire ruby, the same one from Secret of My Excess, on the journey to free Rarity from the moon. He wears it on a cord around his neck. During a lucid nightmare induced by Nightmare Rarity, he shows it to the unicorn, asking if she recognizes it; she doesn't, citing it as not being her color, proving Spike's fear that she isn't real. Following his capture, one of the corrupted moon creatures shatters it, though he escapes with the shards. At the end of the storyline, the ponies use their memories of Rarity to restore her, and in the process the ruby's integrity is restored.