sedimentary
Chert (specifically Chalcedony or Flint variation)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brownish-tan to waxen honey; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Brownish-tan to waxen honey
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Brownish-tan to waxen honey; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from groundwater within limestone or chalk beds. Most specimens found in decorative gravel date from the Cretaceous period to the Paleozoic.
Uses & applications
Historically used for stone tools and weapons due to its sharp edges; modernly used in road construction, as an abrasive, and sometimes tumbled for jewelry or landscaping.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it was the primary material for arrowheads and spear points for thousands of years. It is essentially a non-banded form of agate.
Field identification & locations
Identify in the field by its 'waxy' appearance and smooth, curved break patterns. Common in riverbeds and glacial till. To test, it should be hard enough to scratch glass easily.
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