Onyx is a variety of chalcedony cryptocrystalline from the mineral quartz, having a greasy luster. It may be semi-transparent or translucent and is usually white to gray, grayish-blue or some shade of brown, sometimes black.
Other colours have different names. A red chalcedony is known as coral or carnelian, a green variety colored by nickel oxide is called to chrysophrase. Prasiolite is the one with a elementary green. Plasma is a light colored chalcedony, emerald green which is sometimes found with small spots of jasper resembling blood drops; was called blood stone or heliotrope.
Onyx is a more noble stone than marble.