Coal black and Obsidian

Coal black noun - A very dark black.

Obsidian and coal black are semantically related. in black topic. In some cases you can use "Obsidian" instead an adjective phrase "Coal black".

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

Coal black and obsidian are semantically related. Sometimes you can replace phrase "Coal black" with "Obsidian", this terms are similar.