Black Onyx
Chalcedony var. Onyx · Semi-Precious Gemstone
Crystal System
Trigonal (Hexagonal) system; microcrystalline/cryptocrystalline habit
Mineral Group
Silicates (Tectosilicate group, Quartz family)
Chemical Formula
SiO2 (Silicon Dioxide) with carbon or iron impurities
Formation Process
Formed at low temperatures in cavities of volcanic rocks (gas bubbles/vesicles) or hydrothermal veins
About This Crystal
Jet-black, opaque beads with a smooth, glass-like (vitreous) luster. These specific beads are faceted into geometric shapes which enhance light reflection on the surface despite the stones deep opacity.
Physical Characteristics
Color: Solid black; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.65–2.66. It is non-magnetic and brittle in nature.
Optical Properties
Refractive Index: 1.544–1.553; Birefringence: 0.009; No pleochroism due to opacity. Usually shows a dull or inert reaction to UV light.
Hardness & Durability
Origin Region
Brazil, India, Madagascar, Uruguay, and USA
Hardness & Durability
Mohs Hardness: 6.5–7. Good toughness and excellent durability for daily wear in jewelry like bracelets and rings.
Care & Maintenance
Clean with warm soapy water and a soft brush. Avoid ultrasonic cleaners if the stone has been dyed (common for black onyx). Keep away from harsh chemicals and store separately to prevent scratching softer materials.
Rarity & Value
Relatively common; very affordable. Rarity and value increase for larger, natural (non-dyed) specimens with high polish. Note: Almost all solid black onyx in the jewelry market is gray chalcedony that has been sugar-treated and acid-carbonized to achieve the deep black color.
Special Characteristics
When not solid black, it typically features parallel banding (distinguishing it from Agate's curved banding). These beads are intentionally faceted to create a sparkling 'diamond-cut' effect on an opaque surface.
Lore & History
Historically used in Ancient Egypt for bowls and pottery, and in Rome for cameos and seal rings because it does not stick to wax. Metaphysically, it is traditionally associated with the Root Chakra, believed to offer protection, grounding, and relief from emotional stress or grief.