sedimentary
Black Jasper
Silicon dioxide (SiO2) with carbonaceous impurities
Hardness: 6.5-7 Mohs; Color: Jet black to dark grey; Luster: Vitreous to waxy (when polished); Structure: Microcrystalline; Cleavage: None; Fracture: Conchoidal.
- Hardness
- 6
- Color
- Jet black to dark grey
- Luster
- Vitreous to waxy (when polished)
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Physical properties
Hardness: 6.5-7 Mohs; Color: Jet black to dark grey; Luster: Vitreous to waxy (when polished); Structure: Microcrystalline; Cleavage: None; Fracture: Conchoidal.
Formation & geological history
Formed through the infiltration of silica-rich fluids into sedimentary or volcanic rock beds, often containing carbon or magnetite impurities that provide the black pigmentation. It is a form of chalcedony.
Uses & applications
Commonly used for lapidary work, jewelry (beads, cabochons), as a 'touchstone' for testing precious metals, and for decorative landscaping.
Geological facts
Black jasper has a long historical use as a 'basanite' or touchstone; by rubbing gold or silver on its surface, the streak left behind could determine the purity of the metal based on color comparison.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife), opaque density, and smooth conchoidal fracture. Often found as water-worn pebbles in riverbeds or coastal areas.
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