Black Onyx
Chalcedony var. Onyx · Semi-Precious Gemstone / Decorative Stone
Crystal System
Trigonal (Hexagonal crystal system), Microcrystalline/Cryptocrystalline habit
Mineral Group
Silicates (Tectosilicate subgroup; Quartz group)
Chemical Formula
SiO2 (Silicon Dioxide) with carbon or iron sulfide inclusions
Formation Process
Sedimentary/Hydrothermal; formed by the deposition of silica in gas cavities in lavas, resulting in the distinctive parallel bands typical of chalcedony.
About This Crystal
An opaque, jet-black stone with a smooth, vitreous to waxy luster. While natural onyx typically has parallel bands, solid black onyx is often achieved through treatment. The specimen shown is a polished, ovoid-shaped tumble stone approximately 1.5 cm in diameter.
Physical Characteristics
Color: Deep black; Streak: White; Fracture: Conchoidal; Specific Gravity: 2.65–2.67; Tenacity: Brittle; Fluorescence: Usually inert.
Optical Properties
Refractive Index: 1.544–1.553; Birefringence: 0.009 (often not measurable in opaque varieties); Optical Character: Uniaxial positive; Diaphaneity: Opaque.
Hardness & Durability
Origin Region
Notable sources include Brazil, India, Madagascar, Uruguay, and Mexico; typically found in gas cavities (vesicles) of volcanic rocks.
Hardness & Durability
Mohs Hardness: 6.5–7; Toughness: Good (very durable for jewelry); Stability: High, though may be sensitive to strong acids.
Care & Maintenance
Clean with warm soapy water and a soft brush; avoid ultrasonic cleaners if the stone has been dyed; store separately to prevent scratching softer stones or being scratched by diamonds.
Rarity & Value
Common; very affordable. Value is determined by the uniformity of color and the quality of the polish. Most solid black onyx on the market is gray chalcedony that has been dyed.
Special Characteristics
Typically lacks the chatoyancy or play of color found in other quartz varieties; notable for its ability to take an exceptionally high, glass-like polish.
Lore & History
Used since antiquity for seals, cameos, and intaglios. In ancient Rome, it was highly valued for signet rings because it did not stick to wax. Metaphysically, it is traditionally associated with protection, grounding, and the Root Chakra.