sedimentary

Chert (Flint)

Chert (Cryptocrystalline silicon dioxide, SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Dark grey to black core with a light grey/white weathered cortex; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 6
Luster: Dull to waxy

Identified June 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the replacement of limestone or chalk by silica-rich fluids, or as nodules in sedimentary basins. These specimens in the St. Petersburg region (59.8°N) often originate from Carboniferous or Ordovician carbonate deposits and were redistributed by glacial activity.

Uses & applications

Historically used for stone tools and weapons due to its sharp edges; today used as abrasives, in road construction, and as a collector specimen.

Geological facts

Darker varieties are specifically called flint. This specimen shows a thick white weathered 'cortex' which is common for nodules found in soil or glacial till. It is chemically identical to quartz but has a different microscopic structure.

Field identification & locations

Identify by its waxy luster on fresh breaks and its ability to scratch glass. It produces sparks when struck against steel. Highly common in the Leningrad Oblast and surrounding Baltic regions due to local glacial drift.

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