Onyx and Pitch-dark

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

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

Pitch-dark adjective - Being without light or without much light.

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