sedimentary
Chert (or Chalcedony)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 Mohs; Color: Brownish-tan with semi-translucent areas; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 7 Mohs
- Color
- Brownish-tan with semi-translucent areas
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs; Color: Brownish-tan with semi-translucent areas; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.6-2.7.
Formation & geological history
Formed by the precipitation of silica from groundwater or as a biochemical specimen from the remains of silica-shelled organisms in marine environments. Often found in nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons (flintknapping); currently used in road surfacing, construction aggregates, and occasionally as lapidary material for jewelry.
Geological facts
Chert is one of the most durable rocks and was essential to early human survival for making spearheads and arrowheads because of its sharp, conchoidal fracture edges.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and its tendency to break into smooth, curved surfaces. Common in riverbeds and sedimentary outcrops.
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