Obsidian and Pitch black

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

Pitch black and obsidian are semantically related In some cases you can replace phrase "Pitch black" with "Obsidian", this terms are similar.

Pitch black noun - A very dark black.

Obsidian and pitch black are semantically related in black topic. Sometimes you can use "Obsidian" instead an adjective phrase "Pitch black".