Also known as sardonyx, onyx gemstones are available in three major color combinations: black and white, grey and white, and black.
Other varieties such as red onyx or green onyx are also to be found, even though black is the popular color in this gemstone. The gemstone is also heat-treated sometimes or made to undergo other treatments, to enhance their appearance.
Onyx is rated at 7 out of 10 on the Mohs’ scale of hardness, however, it is still prone to scratches or chips if you do not care for the gem well. It is also worth noting that there are fake onyx stones available in the market as well.
Agate is a gem that has similar physical properties as onyx. Hence, it is common to find jewelry like a circular diamond ring, which are actually dyed agate gems being sold as onyx jewelry. Besides, there are many ordinary black stones sold as onyx too. So you need to be careful when buying the gemstone.