The black onyx in your jewelry is likely a clever chemical deception
In short: True onyx is a rare, naturally banded silicate mineral, yet most of what you see on the market is actually common agate treated with sugar and acid. This ancient technique, documented by Pliny the Elder in the first century, transforms ordinary stone into the dark, parallel-banded gem prized for centuries.
Onyx is a cryptocrystalline variety of agate, composed of fine intergrowths of quartz and moganite. Unlike the chaotic patterns found in many agates, true onyx is defined by its parallel, level bands. The name itself derives from the Ancient Greek word for 'fingernail' or 'claw,' a nod to the stone's occasional resemblance to a nail when pink and white bands are present. While historically significant, the term has become a catch-all label for various materials, including softer, carbonate-based stones like banded calcite, which are frequently carved and sold as onyx today.
The allure of 'black onyx' has driven a long history of artificial enhancement. Since Roman times, artisans have used sophisticated methods to alter the appearance of chalcedony. Pliny the Elder, a first-century naturalist, detailed techniques such as boiling stones in sugar solutions followed by treatment with sulfuric or hydrochloric acid. This process carbonizes the sugar absorbed into the stone's outer layers, creating the deep black color that is rarely found in nature. Today, most black onyx sold commercially remains the product of these or similar dyeing and heating treatments.
Beyond its decorative use in jewelry and cameos, onyx has held a storied place in human culture. It was used for pottery in Egypt's Second Dynasty and served as a protective amulet for Roman soldiers, who believed sardonyx engraved with Mars would grant courage. In the 20th century, the stone transitioned into high architecture and design; Mies van der Rohe famously utilized slabs of onyx for a shimmering wall in the 1930 Villa Tugendhat. Whether used for Renaissance eloquence or as a base for Ferdinand Preiss's art deco sculptures, the stone's cultural footprint is as layered as its physical structure.
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- Summary by Intellect · source: Onyx
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