sedimentary
Chert (Agatized/Chalcedony variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: Tans, grays, and whites; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7.
- Hardness
- 6
- Color
- Tans, grays, and whites
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Tans, grays, and whites; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7.
Formation & geological history
Formed through the precipitation of silica-rich fluids within sedimentary rock cavities or by the replacement of calcium carbonate. This chemical sedimentary rock usually originates in marine environments or volcanic ash deposits.
Uses & applications
Historically used for stone tools (arrowheads). Modernly used in semi-precious jewelry (when polished), lapidary art, and as decorative gravel.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is often found as nodules inside limestone or chalk formations. If it exhibits concentric banding, it is specifically referred to as Agate.
Field identification & locations
Identify in the field by its smooth, waxy surface and conchoidal (shell-like) fracture patterns. It is extremely hard and will scratch glass. Commonly found along riverbeds, beaches, and limestone outcrops.
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