sedimentary
Chert (Nodular Chert)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Reddish-brown, tan, or gray; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Reddish-brown, tan, or gray
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Reddish-brown, tan, or gray; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids or by the replacement of calcium carbonate in limestone by silica. Common in marine deposits ranging from Precambrian to Cenozoic ages.
Uses & applications
Historically used for stone tools (arrowheads). Modernly used in road construction, as an abrasive, and sometimes in landscaping or basic lapidary work.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is one of the most durable rocks on Earth and survives intense weathering, which is why it's often found as river pebbles.
Field identification & locations
Identify in the field by its extreme hardness (will scratch glass and steel), smooth conchoidal (shell-like) fracture edges, and waxy texture. Frequently found in limestone outcrops or as river bed-load.
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