Onyx usually refers to a black-and-white banded variety of agate (cryptocrystalline quartz) although it has also been applied to monochromatic agates with dark and light parallel bands. Banded brown and white agate, however, is usually referred to as sardonyx. Since onyx is not a mineral name but rather a variety, it is often erroneously applied to black and white banded material, such as ‘Mexican Onyx” which is actually limestone.
Banded Onyx
The bands in onyx are one of its most distinctive features. They are created as silica is deposited in lava gas cavities. Over time, these cavities fill with silica-rich solutions and crystallize into chalcedony.
The bands form because of the chemical composition and impurities in the solution and the conditions when they are deposited. Black onyx derives its color from the presence of iron, while the white layers typically, but not always, have traces of carbonate present. The bands are usually parallel and can be uniform in thickness or vary significantly. The degree of banding can range from thin, delicate lines to broad, prominent stripes.
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Where is Onyx Found
Onyx deposits are found worldwide including Brazil, Mexico, India and the United States. The formation of onyx is intricately linked to silica-rich volcanic rocks such as basalt and rhyolite. Onyx is commonly found in association with other materials such as agate, chalcedony and jasper.
Protective Qualities
In various cultures, the bands in onyx are thought to symbolize balance, protection and harmony. The contrasting layers are often interpreted as representing dualities, such as light and dark or good and evil.
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Onyx Uses
Jewelry makers and artisans often choose pieces of onyx with the most striking and visually appealing bands. In the home, onyx is used in the construction of countertops, backsplashes and wall cladding and decorative objects such as sculptures, vases and bowls.
Its Mohs Hardness and durability make onyx suitable for manufacturing precision tools, grinding wheels and cutting equipment. Onyx is utilized in the production of laboratory equipment, optical lenses and components for electronics and technology.
History & Culture
Onyx was highly prized in ancient Egypt and was used extensively in jewelry, amulets and religious artifacts. Ancient Romans and Greeks used onyx to create intricate cameos and intaglios. In Islamic architecture, the banding in onyx was used to add decorative elements to religious structures. Onyx has been revered in various Asian cultures, where it was valued for its beauty and perceived metaphysical properties. Onyx is also the birthstone for July.
This story about onyx appeared in Rock & Gem magazine. Click here to subscribe. Story by Dr. Stuart Mills and Pam Freeman.
