Onyx is a natural mineral that's used for a wide variety of different purposes some of which include kitchen worktops, fireplace surrounds, household furniture, ornaments and the finest grade material is cut and polished for gemstones. It's most accurately described as a cryptocrystalline variety of silicon dioxide or an agate that's made up of bands of quartz and chalcedony. Due to its contrasting colours and attractive banding the Romans used it to make seals and cameos. They considered it to be an ideal lapidary material and it was often dyed black.
In his encyclopedia Naturalis Historia (circa 77-79 AD), the Roman author and naturalist Pliny the Elder wrote about techniques that were used to dye onyx. Although it does occur naturally, black onyx is extremely rare hence most of the material that's available to buy today is dyed. As well as being dyed black, stones are also dyed many other colours because onyx is known to retain artificial colours particularly well.
The history of onyx can be traced back even further and is believed to have been one of the gemstones in the High Priest's breastplate, a religious garment worn during biblical times by the Jewish High Priest when presenting himself to God.
In crystal healing onyx is said to be useful for dealing with grief, strengthening self control and to stimulate the power of good decision making. It is also beneficial for self confidence, stamina, phobias, controlling sexual desire and for balancing the yin yang as well as encouraging happiness and luck.
Onyx which is the birthstone for the month of July along with ruby is a relatively hard stone which grades 7 on Mohs scale of mineral hardness.
Wikipedia on onyx