Onyx and Obsidian

Onyx noun - A chalcedony with alternating black and white bands; used in making cameos.

Obsidian and onyx are semantically related in black topic. In some cases you can use "Obsidian" instead an adjective "Onyx".

Obsidian noun - Acid or granitic glass formed by the rapid cooling of lava without crystallization; usually dark, but transparent in thin pieces.

Onyx and obsidian are semantically related in black topic. You can use "Onyx" instead an adjective "Obsidian", if it concerns topics such as dark.