sedimentary
Chert with Desert Varnish
Cryptocrystalline Quartz (SiO2) with Manganese/Iron oxide coating
Hardness: 7 on Mohs scale; Color: Dark brown to black outer coating, lighter interior; Luster: Waxy to dull; Crystal structure: Microcrystalline; Conchoidal fracture; Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Dark brown to black outer coating, lighter interior
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark brown to black outer coating, lighter interior; Luster: Waxy to dull; Crystal structure: Microcrystalline; Conchoidal fracture; Specific gravity: 2.65.
Formation & geological history
Formed as a microcrystalline sedimentary rock in marine or terrestrial environments. The dark 'varnish' is a thin coating formed over thousands of years by wind-blown dust and biochemical processes in arid environments.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to conchoidal fracture. Today used as decorative landscaping stone or as a study specimen for desert geomorphology.
Geological facts
Desert varnish grows incredibly slowly, often less than a millimeter every few thousand years. It is often used by archaeologists to date petroglyphs carved into the dark surface.
Field identification & locations
Identify by its very hard, quartz-like interior and dark, manganese-rich outer crust. Found commonly in arid regions like the American Southwest. It will scratch glass easily.
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