Question: how to tell the difference between jet and black onyx?
Answer: Onyx. ... Between 2 similar sized pieces of onyx & obsidian…which can look very similar, the onyx will be heavier b/c it has an actual crystalline structure, whereas obsidian does not…with its random molecules & air bubbles. Onyx tends to have a slightly duller finish, but can polish up to be quite glassy!