mineral
Black Agate / Onyx (Tumbled Freeform)
Chalcedony (Silicon Dioxide, SiO2)
AI-generated identification · may be incorrectHardness: 6.5-7 on the Mohs scale; Color: Deep glossy black with subtle cloudy/milky dendritic or translucent grey undertones; Luster: Vitreous to waxy (high polish); Crystal system: Trigonal (cryptocrystalline); Specific Gravity: 2.58-2.64.
- Hardness
- 6
- Luster
- Vitreous to waxy (high polish)
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Physical properties
Hardness: 6.5-7 on the Mohs scale; Color: Deep glossy black with subtle cloudy/milky dendritic or translucent grey undertones; Luster: Vitreous to waxy (high polish); Crystal system: Trigonal (cryptocrystalline); Specific Gravity: 2.58-2.64.
Formation & geological history
Formed through the deposition of silica-rich groundwater in cavities, vesicles, or fissures within volcanic rocks or sedimentary strata. Many deep black agates are naturally occurring, though commercially they are frequently enhanced or dyed using traditional chemical treatments.
Uses & applications
Popular for decorative displays, carving, lapidary cabochons, metaphysical crystals, and jewelry pieces like pendants and rings.
Geological facts
Black chalcedony has been used since antiquity in Greece and Rome for protective amulets and signet rings. Natural un-dyed completely solid black agate is relatively rare compared to banded onyx or dyed agate.
Field identification & locations
Identified by its high waxy-to-glassy polish, conchoidal fracture if chipped, high hardness (cannot be scratched by a steel nail), and cool thermal feel. Frequently sourced from Brazil, Madagascar, Uruguay, and India.
