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.
Uses (non-BC) Edit
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
Blood Crystals are different from other gems, as they are used as a form of currency.