Coal-black and Obsidian

Coal-black adjective - Of the blackest black; similar to the color of jet or coal.

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 in black topic. You can use "Coal-black" instead an adjective "Obsidian".