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 (and penalized attunement to the opposing element) while the equipment is worn or wielded. This stacks with attunement from passive skills.