sedimentary

Chert (specifically Blue Chert or Flint)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified February 26, 2026More sedimentary →

Explore Chert (specifically Blue Chert or Flint) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

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.