sedimentary
Chert (Jasper/Ironstone variety)
Silicon Dioxide (SiO2) with iron oxide inclusions
Hardness: 6.5-7 Mohs scale; Color: Reddish-brown, earthy tan; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Reddish-brown, earthy tan
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 Mohs scale; Color: Reddish-brown, earthy tan; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary environments. Frequently found in Precambrian and Paleozoic formations, often enriched with hematite or goethite giving it the brown-red hue.
Uses & applications
Historically used for stone tools and arrowheads; currently used as landscaping stone, construction aggregate, and occasionally as lapidary material for polishing.
Geological facts
Chert is so durable that it often survives long after the surrounding rock has weathered away. The reddish-brown colors are usually a result of trace amounts of iron oxidation within the silica matrix.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its smooth, shell-like (conchoidal) fracture surfaces, and its lack of visible grains. Found worldwide in riverbeds and limestone outcrops.
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