sedimentary
Chert (often called Flint)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs); Color: Tan, brownish, or gray; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Tan, brownish, or gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan, brownish, or gray; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. Most common in bedrock dating from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges. Modernly used in road construction as aggregate, landscaping, and flint-striking fire starters.
Geological facts
Chert was one of the most important minerals for early humans. Its conchoidal fracture allows it to break into edges that are sharper than a surgical steel scalpel.
Field identification & locations
Identify in the field by its smooth, shell-like fracture pattern and ability to scratch glass. Found commonly in limestone or chalk deposits globally, particularly in the UK, USA, and France.
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