The Green Gem is an item used to craft two Ancient items. It can be used to craft the Construction Amulet, which raises Sanity as well as halves the resource cost of crafting items, and the Deconstruction Staff, which can be used to destroy structures/items and receive all of the resources used in their construction (barring Gems).
Spazmatism can be killed every 15 days for a guaranteed Green Gem.
Green Gems can be gathered in the Ruins from Cave Holes by using The Lazy Forager. The Ruins can be regenerated by killing the Ancient Fuelweaver, so every time the Ancient Fuelweaver is killed, the player can regenerate new gems.
Obtained by inserting the Gem into the Cratered Moonrock.
Tips
Because Green Gems are renewable, the items it can be crafted into can be utilized to make nearly any other item in the game renewable, by crafting something with the amulet at half-price and then deconstructing it with the Deconstruction Staff for a full return.
Trivia
The Green Gem uses the Red Gem's texture, only flipped and colored green.
Despite also being green, the gem in Wormwood's body came from the moon.
Attention: The following categories are only for the players look up the page based on their first impression, they have nothing to do with the actual categories/behavior/function/tags of these items