Onyx is part of the chalcedony family of colored quartz, which includes agate, cornelian and jasper. The striking black and often-occurring crisp lines within onyx make it especially popular for jewelry. Because the lines can form in many different ways, each piece of onyx has a unique appearance. In some cases, you cannot see the lines in onyx at all. Onyx is also popular for cameos - when an image is carved into onyx, the color of the next band shows through. Onyx is opaque, meaning no light shines through it. Therefore, it is usually smooth-cut into a cabochon. Onyx usually features white or grey lines, but it can also exhibit red to brownish-red lines.
Onyx was supposed to drive away evil, to cool the passions of love, and promote independence between lovers. Some people even believed it was a symbol of discord, a belief probably suggested by the sharply divided lines in the stone.
Onyx gets its color from silicon dioxide. It has a Mohs hardness rating of 7, making it appropriate for all types of jewelry.