Black Opal
Amorphous Hydrous Silica var. Black Opal · Precious Gemstone
Crystal System
Amorphous (Non-crystalline/Microcrystalline); habit is typically massive, vein-filling, or botryoidal.
Mineral Group
Silicates (Tectosilicate / Mineraloid)
Chemical Formula
SiO2 · nH2O (Silicon dioxide with varying water content)
Formation Process
Sedimentary/Hydrothermal; formed as silica-rich water seeps into cracks in the earth, then solidifies as the water evaporates over millions of years.
About This Crystal
An opaque to translucent dark-bodied stone with a vitreous to waxy luster. The specimen shows a dark matrix with distinct 'play-of-color' or 'fire' presenting as flashes of blue/green against a dark background.
Physical Characteristics
Color: Dark grey to black body; Streak: White; Cleavage: None; Fracture: Conchoidal to uneven; Specific Gravity: 1.98–2.25; Fluorescence: Often inert but can show green or white fluorescence under UV.
Optical Properties
Refractive Index: 1.450 (+/- .020); Birefringence: None (isotropic); Optical Phenomenon: Play-of-color (diffraction of light through silica spheres).
Hardness & Durability
Origin Region
Lightning Ridge (Australia), Ethiopia (Welo), and Virgin Valley (USA); found in sedimentary rocks or volcanic cavities.
Hardness & Durability
Mohs Hardness: 5.5 to 6.5; Toughness: Poor to fair (brittle and prone to cracking/crazing); Stability: Sensitive to heat and sudden temperature changes.
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoid ultrasonic or steam cleaners. Store in a cool, slightly humid environment; avoid prolonged exposure to direct sunlight or harsh chemicals.
Rarity & Value
Very Rare; Black opal is the most valuable variety of opal due to the dark base enhancing the fire. Values range from hundreds to thousands of dollars per carat depending on the intensity of 'fire'.
Special Characteristics
Play-of-color (iridescence); may contain 'potch' (common opal) as part of the matrix or show color-change depending on the light source angle.
Lore & History
Historically known as the 'Eye Stone' in ancient cultures; black opal is traditionally associated with the crown and root chakras, believed to provide protection and enhance emotional depth.