Onyx and Pitch black

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

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

Pitch black noun - A very dark black.

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