sedimentary
Chert (specifically Blue Chert or Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray to deep bluish-gray with a tan/white limestone cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline (indistinct crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray to deep bluish-gray with a tan/white limestone cortex
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray to deep bluish-gray with a tan/white limestone cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline (indistinct crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed as nodules or layered deposits in limestone or chalk beds. It originates from the accumulation of silica-rich organic remains (like sponge spicules or radiolarians) that dissolve and reprecipitate in sedimentary environments. Can range from Precambrian to recent age.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used in road construction as aggregate, in jewelry as cabochons, and as a landscaping stone.
Geological facts
Because of its conchoidal fracture, it breaks into extremely sharp edges, making it the primary material for prehistoric arrowheads and spear points. Variations in color are caused by trace inclusions of minerals like iron or organic matter.
Field identification & locations
Identify by its 'glassy' internal texture when broken compared to its rough, chalky outer rind (cortex). It will easily scratch glass and displays curved, shell-like fracture patterns. Commonly found in limestone outcrops or riverbeds globally.
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