Black Onyx
Chalcedony var. Onyx · Semi-Precious Gemstone
Crystal System
Trigonal/Hexagonal system with a microcrystalline (cryptocrystalline) habit
Mineral Group
Silicates (Tectosilicate subgroup; Quartz group)
Chemical Formula
SiO2 (Silicon Dioxide) with trace iron and carbonaceous inclusions
Formation Process
Sedimentary or Volcanic; forms from the slow deposition of silica in gas cavities (vesicles) in lava at low temperatures.
About This Crystal
Opaque, deep black material with a smooth, vitreous (glassy) luster. When polished and faceted as seen in the image, it shows sharp reflections and a dense, saturated black color without visible banding.
Physical Characteristics
Color: Jet Black; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.65-2.67; Tenacity: Brittle; Fluorescence: Generally inert.
Optical Properties
Refractive Index: 1.544-1.553; Birefringence: up to 0.009; Pleochroism: none; Optical character: Uniaxial positive.
Hardness & Durability
Origin Region
Notably Brazil, India, Madagascar, and Uruguay; typically found in gas cavities in volcanic rocks.
Hardness & Durability
Mohs Hardness: 6.5 to 7; Toughness: Good (very durable for daily wear); Stability: Stable to light and most household chemicals.
Care & Maintenance
Clean with warm soapy water and a soft brush; avoid harsh chemicals or ultrasound if the stone has been dyed (most black onyx is dyed). Store separately to avoid scratching silver settings.
Rarity & Value
Common; very affordable. Value is determined by the quality of the polish and the depth of the black color. Natural solid black onyx is rare; most commercial black onyx is dyed chalcedony.
Special Characteristics
Cryptocrystalline structure means crystals are too small to be seen under a normal microscope. Often used for intaglios and cameos due to its fine texture.
Lore & History
Historically used in ancient Egypt and Rome for seals and jewelry. Symbolically associated with protection, grounding, and self-control. Traditionally linked to the Root Chakra.