Red Gems are a resource introduced in Aberration and also available on Valguero, Genesis: Part 1, Crystal Isles, Genesis: Part 2, Lost Island, and Fjordur. They are used for crafting various items.
Resource Map/Crystal Isles - ARK Official Community Wiki
Halls of the Reaper (Aberration) - ARK Official Community Wiki
Steam Community :: Guide :: HOW TO GET RED GEMS WITHOUT A HAZARD SUIT IN ARK ABERRATION & HOW TO BUILD A ROLL RAT GEM FARM
Red Gem, Don't Starve Wiki
The Unknown (Valguero) - ARK: Survival Evolved Wiki
Aberration - ARK Official Community Wiki
Box o' Chocolates - ARK: Survival Evolved Wiki
Susecdurus, Additional Creatures Wiki
Official ARK Wiki - Feedback - General Discussion - ARK - Official Community Forums
Elemental Vault (Aberration) - ARK Official Community Wiki
Gacha Crystal (Extinction) - ARK: Survival Evolved Wiki
Ascension: Deckbuilding Game, Board Game
Red Gem - ARK Official Community Wiki
Ebenus Astrum - ARK Official Community Wiki