Onyx is formed of bands of chalcedony in alternating colors. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and moganite. Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates.
Onyx is rare, beautiful, and unique but requires extra care if used as a countertop. It’s softer and more susceptible to etching than other natural stone, like granite, but if you require that one of a kind look, and you seal the onyx and regularly care for it you will be rewarded with a beautiful showplace in your kitchen.