sedimentary
Chert (Chalcedony/Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale; Color: Gray, tan, brown, and translucent cream; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Color
- Gray, tan, brown, and translucent cream
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on the Mohs scale; Color: Gray, tan, brown, and translucent cream; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed via the accumulation of silica-rich microfossils (like radiolaria or diatoms) or through the chemical precipitation of silica in groundwater within sedimentary host rocks. Common in limestone or dolostone formations.
Uses & applications
Used historically for stone tools (knapping) due to its sharp edges; modern uses include aggregate in construction, abrasives, and as semi-precious lapidary material for jewelry and cabochons.
Geological facts
Chert has been used by humans for over two million years to create tools. Flint is a variety of chert often associated with chalk or marly limestone. When struck against steel, chert can produce sparks, leading to its historical use in flintlock firearms.
Field identification & locations
Field identification involves looking for a waxy luster, conchoidal (shell-like) fracture patterns, and testing for high hardness (it will scratch glass). It is often found as nodules or layers within sedimentary rock outcrops.
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