Black Onyx
Chalcedony (cryptocrystalline Quartz) var. Onyx · Semi-Precious Gemstone / Decorative Stone
Crystal System
Trigonal (specifically Microcrystalline/Cryptocrystalline); Habit: Massive, botryoidal, or rounded tumbled stone
Mineral Group
Silicates (Tectosilicate)
Chemical Formula
SiO2 (Silicon Dioxide) with carbon or iron impurities
Formation Process
Hydrothermal/Sedimentary: Precipitates from silica-rich fluids at low temperatures in the cavities of igneous rocks
About This Crystal
Opaque to slightly translucent black stone with a waxy to vitreous luster. This specimen appears as a small, smooth, tumbled pebble with a matte-to-satin finish and deep black pigmentation.
Physical Characteristics
Color: Jet black; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.65–2.67; Fluorescence: Usually none; Tenacity: Brittle but tough due to interlocking structure.
Optical Properties
Refractive Index: ~1.544–1.553; Birefringence: 0.009 (usually not measurable in opaque varieties); Singly or doubly refractive (Aggregates); No pleochroism.
Hardness & Durability
Origin Region
Brazil, India, Madagascar, Uruguay, and USA; formed in gas cavities (vesicles) of volcanic rocks
Hardness & Durability
Mohs Hardness: 6.5–7; Toughness: Excellent; Very durable for jewelry and daily wear; resistance to common household acids.
Care & Maintenance
Safe to clean with warm soapy water and soft brush; avoid ultrasonic or steam cleaners if the stone has been dyed; store away from harder gems like diamond or sapphire to prevent scratching.
Rarity & Value
Common; Value is generally low; Highly saturated jet black specimens are most desirable; Note: Natural solid black onyx is rare, most commercial black onyx is dyed chalcedony.
Special Characteristics
Parallel banding (if present, distinguishing it from Agate's curved bands); opaque nature; lack of visible grains; thermal stability.
Lore & History
Historically used in ancient Egypt and Rome for seals and cameos. In metaphysical lore, it is considered a 'grounding stone' associated with the Root Chakra, believed to provide strength, protection, and support during emotional stress.