Black onyx is a captivating gemstone often seen in jewelry. Understanding its characteristics, natural formation, and common treatments helps distinguish it from other dark gemstones.
The Distinctive Black of Onyx
Black onyx is a variety of chalcedony, a microcrystalline form of quartz, characterized by a deep, uniform black color and opaque nature. It typically features a smooth, polished surface with a vitreous, glass-like luster. This intense black results from impurities like carbon and iron, or through enhancement processes. A solid, consistent black is the desired aesthetic for black onyx, distinguishing it from other banded onyx varieties that display multiple colors.
Natural Characteristics and Color Treatments
Natural onyx often forms with contrasting layers or bands of various colors. While naturally occurring solid black onyx exists, it is exceptionally rare. Most commercially available black onyx achieves its deep, consistent black through a treatment process, typically dyeing gray or white chalcedony. This treatment often includes soaking the stone in a sugar solution, followed by immersion in sulfuric or hydrochloric acid to carbonize the sugar, resulting in the dark hue. These color enhancements are stable and widely accepted within the gemstone industry.
Comparing Black Onyx to Other Dark Gemstones
Black onyx is often confused with other dark gemstones, but visual cues can help differentiate them. It is opaque, possesses a consistent deep black, and often has a smooth, vitreous luster. It is generally denser and feels heavier than obsidian and typically measures 6.5 to 7 on the Mohs hardness scale.
Obsidian is a natural volcanic glass with a distinctly glossy, reflective appearance. Thin sections can appear translucent when held to light. It is characterized by conchoidal fractures and is lighter in weight than onyx. Obsidian’s Mohs hardness ranges from 5 to 5.5, making it softer than black onyx.
Black tourmaline is a crystalline mineral often exhibiting visible striations or growth lines. Its luster can range from waxy to dull, though polished pieces may achieve a glossy finish. While typically deep black, some black tourmaline might show slight translucency in very thin sections.
Jet, an organic material formed from fossilized wood, is notably lightweight and often feels warm to the touch. Unlike black onyx, jet does not achieve as high a polish, often presenting a more matte or waxy surface.