Black Onyx
Quartz var. Onyx (Chalcedony) · Semi-Precious Gemstone
Crystal System
Trigonal (Cryptocrystalline); habit is massive or microcrystalline, showing a smooth, tumbled finish in this specimen.
Mineral Group
Silicates (Tectosilicate)
Chemical Formula
SiO2 (Silicon Dioxide)
Formation Process
Hydrothermal/Sedimentary; formed by the deposition of silica from groundwater in gas cavities of volcanic rocks at low temperatures.
About This Crystal
A tumbled, opaque black gemstone with a smooth, vitreous to waxy luster. The specimen is roughly 2cm in length, exhibiting a uniform deep black color without visible banding.
Physical Characteristics
Color: jet black; Streak: white; Cleavage: none; Fracture: conchoidal (shell-like); Specific Gravity: 2.65-2.66; Fluorescence: typically inert.
Optical Properties
Refractive Index: 1.543-1.553; Birefringence: up to 0.009; typically opaque in thick sections but may show translucent edges under intense light.
Hardness & Durability
Origin Region
Brazil, India, Madagascar, and USA; typically forms in volcanic gas cavities (vesicles).
Hardness & Durability
Mohs Hardness: 6.5 to 7; Toughness: good; very durable for most jewelry types but can be scratched by topazes or sapphires.
Care & Maintenance
Clean with warm soapy water and a soft brush; avoid ultrasonic cleaners if the stone is dyed; store away from harder gems like diamonds.
Rarity & Value
Common; value is determined by size and polish quality. Most solid black onyx is produced by treating gray chalcedony with a sugar-acid chemical solution.
Special Characteristics
Cryptocrystalline structure; while natural black onyx exists, it is rare, and carbon-staining treatments are the industry standard for obtaining uniform black color.
Lore & History
Historically used in Rome for seals and cameos. In metaphysical traditions, it is believed to be a grounding stone that provides strength and protection against negative energy.