sedimentary
Black Agate
Silicon Dioxide (SiO2) with carbon/iron inclusions
Hardness: 6.5-7 Mohs; Color: Black with grey or white banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64
- Hardness
- 6
- Color
- Black with grey or white banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5-7 Mohs; Color: Black with grey or white banding; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64
Formation & geological history
Formed in volcanic and metamorphic rocks as a result of silica-rich groundwater filling cavities or vesicles, often throughout millions of years of deposition.
Uses & applications
Used primarily in jewelry (beads, cabochons), ornamental carvings, and as touchstones in metaphysical practices.
Geological facts
While true black agate exists, many specimens on the commercial market are heat-treated or dyed chalcedony to enhance the deep black saturation.
Field identification & locations
Identify by its waxy luster, conchoidal fracture when chipped, and typical banding patterns. Commonly found in Brazil, India, and the United States.
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