Onyx is a variety of the mineral chalcedony, a microcrystalline form of quartz. This material is defined by its distinct straight, parallel banding, typically seen as alternating layers of black and white. True siliceous onyx must be distinguished from “onyx marble,” which is a softer carbonate material known as calcite. This guide focuses on identifying the authentic, quartz-based variety of onyx.
Visual Properties and Surface Examination
True onyx possesses a luster that ranges from waxy to dull when unpolished, taking on a glass-like shine after being buffed. The most telling characteristic is the arrangement of its coloration, which occurs in straight, parallel bands or layers. This linear structure differentiates onyx from other forms of banded chalcedony, such as agate, which typically exhibits curved or concentric banding.
Color uniformity is an important visual clue, especially for black onyx, the most popular variety. Naturally occurring solid black onyx is rare, and the majority sold today is actually gray chalcedony or agate that has been permanently dyed to achieve a deep, uniform color. Naturally banded material will show subtle, organic variations in color saturation between the layers. If a stone appears jet black with an unnatural, paint-like uniformity, it is most likely a dyed stone, which is an accepted treatment for genuine chalcedony.
A visual inspection should also search for any structural imperfections inconsistent with a natural stone. The presence of tiny, spherical air bubbles just beneath the surface is a definitive sign that the material is glass, not natural onyx. Plastic or resin imitations may sometimes retain subtle mold lines or seams, which would never be found on a stone formed naturally. The surface should be smooth and consistently solid, with no signs of flaking or uneven color application.
Non-Destructive Physical Tests
Real onyx is a hard mineral, registering between 6.5 and 7 on the Mohs scale of mineral hardness, a characteristic shared with quartz. This level of hardness means the stone is resistant to scratching from many common household items. A simple scratch test can be performed on an inconspicuous area using a steel nail or a piece of ordinary glass. Genuine onyx should scratch the glass (Mohs 5.5) but should not be scratched by the steel (Mohs 5.5 to 6.5). If the stone is easily marked or gouged by a steel point, it is softer and not true siliceous onyx.
The stone’s thermal properties are another practical test. Natural stone, including onyx, is a poor conductor of heat, meaning it should feel distinctly cool to the touch, even in a warm room. This coolness is retained for a short time, and the stone warms up very slowly when held in the hand. Conversely, plastic or resin fakes will quickly reach body temperature or feel room temperature immediately upon touching them, lacking the characteristic initial coolness of a genuine stone.
The stone’s diaphaneity, or light transmission, is a final check. True onyx is generally opaque, meaning light cannot pass through it. On very thin edges, it may exhibit slight translucency, but if the entire piece allows light to pass through easily, it suggests the material is likely glass, a highly translucent resin, or a different type of mineral. Testing the density by weighing the stone in your hand can also help, as a piece that feels surprisingly light for its size may indicate a plastic imitation.
Identifying Common Onyx Imitations
Dyed agate or chalcedony is often chemically treated to achieve a saturated black color. While the base material is still quartz, the presence of parallel banding is typical for true onyx. If the stone displays curved, concentric, or irregular banding patterns, it is a banded agate, not true onyx. The dye used to turn the material black is usually stable, but a simple rub with a cotton swab and acetone can occasionally reveal a fake if the color transfers.
Glass or obsidian can be mistaken for black onyx due to its dark appearance. Glass and obsidian are amorphous, meaning they lack the crystalline structure of quartz, and will often exhibit a characteristic conchoidal fracture pattern. Glass imitations may also contain small, trapped air bubbles, which are a definitive sign of artificial origin. Furthermore, glass is considerably softer than onyx, typically falling between 5 and 5.5 on the Mohs scale, and can be easily scratched by a steel point.
Onyx marble is frequently marketed simply as “onyx,” especially for decorative items and carvings. Onyx marble is composed of calcite, a form of calcium carbonate, which is geologically distinct from the quartz-based true onyx. The significant difference is in hardness: calcite is a very soft 3 on the Mohs scale, meaning it can be easily scratched with a copper coin or even a sharp knife. A definitive test to distinguish calcite from true onyx is the acid test, where a tiny, inconspicuous drop of mild acid, such as vinegar, will cause a noticeable fizzing reaction on calcite due to the release of carbon dioxide, while true quartz-based onyx will remain unaffected.