Black Onyx
Quartz var. Chalcedony · Semi-Precious Gemstone
Crystal System
Trigonal (Monoclinic pseudomorphs often reported); typically occurs in a microcrystalline or cryptocrystalline habit, appearing massive or as rounded nodules when naturally weathered or tumbled.
Mineral Group
Silicates (Tectosilicate subgroup)
Chemical Formula
SiO2 (Silicon Dioxide) with trace inclusions of iron and manganese or carbon
Formation Process
Found in volcanic and sedimentary environments. It forms through the deposition of silica from groundwater in gas cavities in lava or from hydrothermal fluids, resulting in its characteristic layered or banded structure.
About This Crystal
The specimen is a tumbled or polished stone, opaque and jet black in appearance. It exhibits a smooth, vitreous (glassy) to waxy luster. In its natural form, it often features parallel banding, though solid black specimens are frequently achieved through treatment.
Physical Characteristics
Color: Opaque black; Streak: White; Cleavage: None (it is a cryptocrystalline variety); Fracture: Conchoidal; Specific Gravity: 2.65–2.67; Tenacity: Brittle but relatively durable due to its dense microcrystalline structure.
Optical Properties
Refractive Index: 1.543–1.553; Birefringence: Up to 0.009 (often not measurable in opaque varieties); Dispersion: Low; Optical Character: Uniaxial positive; Lacks pleochroism due to opacity.
Hardness & Durability
Origin Region
Brazil, India, Madagascar, Uruguay, and various locations in the USA. It typically forms in the gas cavities (vesicles) of volcanic rocks.
Hardness & Durability
Mohs Hardness: 6.5 to 7. It is quite durable and resistant to scratching, making it excellent for daily wear in rings or pendants, although it can chip if struck with force.
Care & Maintenance
Safe to clean with warm soapy water and a soft cloth. Avoid harsh chemicals and ultrasonic cleaners if the stone has been dyed (as many commercial black onyx stones are). Store separately to prevent scratching softer stones.
Rarity & Value
Common. Value is generally low to moderate. Most solid black onyx on the market is originally gray chalcedony that has been stained or heat-treated to achieve a uniform black color, which is a standard and accepted industry practice.
Special Characteristics
When not completely opaque and solid black, it may show parallel banding. It does not typically exhibit fluorescence. Its main draw is its high polish and deep, uniform darkness.
Lore & History
Historically used in Ancient Egypt for pottery and in Rome for cameos and seals because it does not stick to wax. It is traditionally associated with the Root Chakra and is culturally symbolized as a stone for protection, grounding, and self-control.