Onyx is a cryptocrystalline form of quartz, specifically it’s a banded variety of chalcedony, an oxide mineral. That means the crystals contained in this stone are so small that are only vaguely revealed under the microscope, and they are mainly formed of silica. Onyx can have a huge variety of colours, basically any one, excluding some particular shades such as blue and purple, and it usually has bands of colors of white, brown and tan.
This is a stone famously known for its translucent properties and vitreous luster, once polished and treated. Onyx is also believed to have physical and metaphysical properties of energizing and contributing to willpower, increasing self-discipline and other positive qualities. Many other stones are mistakenly sold as Onyx, such as certain types of banded calcite, which are much softer than true Onyx, in fact most of the carved items sold as Onyx nowadays are instead carbonate material.
Industrially, this stone has been used throughout time for jewelry, often cut into cabochon or beads, and for cameo engraved gems. Onyx, as it’s intended to be, scores a solid 7 on the Mohs hardness scale, being formed mainly by silicate, similarly to quartz.
Here is a list of most common stones commercialized as Onyx:
(click on names to open material sheet)
The Stone Wiz 