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 black topic. In some cases you can use "Pitch-black" instead an adjective "Onyx", when it comes to topics like colors.

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

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