There is also Blood Crystals, which are used as in-game currency and can be purchased for real money or found after obtaining tier 2 research in mining.
Note: Selling gems is not recommended. Instead you can trade them with other players or at the Gem Goblin Shop.
Gems are used for...
- Upgrading Tools (and Berserker Amulets/Rings), improving their speed and/or efficiency. You need to socket the gems in order by rarity: Empty -> Sapphire -> Emerald -> Ruby -> Diamond -> Blood Diamond.
- Crafting keys, which are used to open Treasure Chests. The better the key, the more chest slots you have. More chest slots means a higher chance if getting better loot. (see the page on treasure chests for more info)
- Upgrading Rings
- Gem Displays , which are useful for crafting XP and can be sold for some coins, but they are less profitable than Trading.
Blood Crystals are different from other gems, as they are used as a form of currency.