sedimentary
Chert (Nodular)
Chert (SiO2 - Microcrystalline Quartz)
Hardness: 7 (Mohs scale), Color: Varying shades of grey, tan, and black with visible impurities; Luster: Waxy to dull (earthy); Crystal Structure: Hexagonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Luster
- Waxy to dull (earthy)
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Physical properties
Hardness: 7 (Mohs scale), Color: Varying shades of grey, tan, and black with visible impurities; Luster: Waxy to dull (earthy); Crystal Structure: Hexagonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic debris (such as radiolarians or sponge spicules) on the ocean floor, or via chemical precipitation from silica-rich fluids. It often occurs as nodules within limestone or chalk beds. Geological age can range from Precambrian to Holocene.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its conchoidal fracture. Modern uses include road surfacing mineral, decorative landscaping stone, and occasionally as a low-cost gemstone for cabochons.
Geological facts
Chert is so durable that it often survives as 'river gravel' long after the surrounding limestone bedrock has weathered away. Its variety 'flint' was essential to human survival for thousands of years both as a tool and a fire-starter.
Field identification & locations
Identify in the field by its hardness (will easily scratch glass) and its curved, shell-like (conchoidal) fracture pattern. Commonly found in stream beds or within sedimentary rock outcrops. Collectors look for unique banding or fossils trapped within.
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