Onyx (Unrefined Gemstone)

 is a Crafting resource used for the Alchemy skill.

is an Alchemy craftable item. It can be further crafted into cut gems that can be socketed into crafted chest armor and weapons (also at alchemy table). A socketed gem adds attunement to that gem's magical element while the equipment is worn or wielded. This stacks with attunement from passive skills.