sedimentary
Chert (Chert Pebble)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brown, or grayish; Luster: Waxy to dull; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, or grayish
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, or grayish; Luster: Waxy to dull; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from silica-rich groundwater within limestone or chalk beds. Often smooth and rounded due to water erosion (river or beach tumbling).
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; modernly used in road construction, as an abrasive, and small polished specimens for hobbyist collecting.
Geological facts
Chert is so hard it can scratch glass and steel. Because it breaks with a conchoidal (shell-like) fracture, it was one of the most important minerals for early human survival for making fire and weapons.
Field identification & locations
Identify in the field by its hardness (will scratch a knife blade) and its smooth, waxy feel. Commonly found in riverbeds, glacial till, and as nodules within sedimentary rock outcrops globally.
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