Purple 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.
Tumbleweeds have a roughly 0.01% chance to drop Purple Gems when harvested.
Purple Gems are required in large quantities to craft Moon Rock Idols, charge Telelocators, and power Winona's G.E.M.erator (though it is more efficient to use Blue or Red Gems for this purpose since they can be easily acquired on the surface, although killing the Ancient Fuelweaver and clearing the Ruins repeatedly tends to produce an excess of Purple Gems). They're also used for several other character-specific recipes and useful gear such as Nightmare Amulets. See below for a full list.
Obtained by inserting the Gem into the Cratered Moonrock.
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Tips
Thanks to periodic Hound waves, purple gems are renewable, even in the base game, as both Red Hounds and Blue Hounds drop Red and Blue Gems, respectively.
Since the July 30th, 2015 update, they are also renewable by fishing up Sea Worthers, Toy Boats, and Sextants from Flotsam and trading them to Wolly for Purple, Blue, and Red Gems, respectively.
The Purple Gem's crafting description is a reference to Captain Planet.
Purple Gems can be deconstructed using the Deconstruction Staff. However, since they're made using only gems, nothing will be dropped, wasting a Purple Gem.
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