ONYX originates in the dripstone deposits of limestone caverns, where it forms stalactites, stalagmites, and other formations that can fill an entire cavern or void space. It is deposited by gentle, dripping water movement followed by evaporation between drops that deposits calcium carbonate from the water onto the formation, incrementally enlarging formations by thousandths of an inch or less per drop. Thus onyx is also a chemical sedimentary stone, and may envelop terrestrial fossil remains. Although this process of drop-by-drop addition of material does take time, large deposits of onyx begin to mature (filling caverns or fractures) in a relatively short period of geologic time.
Onyx tends to be more crystalline, strongly banded and colored in browns to yellows and clear. It can be translucent, and light-colored varieties sliced thin are used for attractive backlit display panels or even light-admitting windows. Onyx, like most limestone, will recrystallize in time (not a metamorphic process), often enhancing translucency.
The color, veinings, clouds, mottlings, and shadings in marble are caused by substances included in minor amounts during formation. Iron oxides make the pinks, yellows, browns, and reds. Most grays, blue grays, and blacks are of bituminous origin. Greens are caused by micas, chlorites, and silicates.