sedimentary

Chert (Basanite variety) with Quartz Vein

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Black to dark grey with a white/grey vein; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 6
Color: Black to dark grey with a white/grey vein
Luster: Waxy to vitreous

Identified April 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich microfossils or chemical precipitation within marine sediments. The white vein formed later as silica-rich fluids filled a crack in the host rock. Ages range from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (flintknapping); currently used as river rock for landscaping, tumble-polishing for hobbyists, and occasionally as a base for touchstone testing of precious metals.

Geological facts

Black chert is often referred to as 'Lydite' or 'Touchstone' because it was historically used to test the purity of gold by observing the color of the streak left on its surface.

Field identification & locations

Identify in the field by its hardness (will scratch glass), smooth waxy texture, and unique conchoidal (shell-like) fracture pattern. Often found as pebbles in riverbeds or beaches.

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