sedimentary
Chert (specifically Iron-stained or Jasperoid Chert)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brown, tan, and black mottled, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, tan, and black mottled, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from silica-saturated fluids in limestone or marine environments. This specimen shows darkening likely due to iron or manganese inclusions.
Uses & applications
Used historically for stone tools and arrowheads due to its conchoidal fracture. Modern uses include road surfacing, decorative landscaping, and occasionally lapidary work if highly colorful.
Geological facts
Chert is so durable that it often survives as a 'river stone' long after the surrounding limestone bedrock has weathered away. It is virtually immune to chemical weathering.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture surfaces. Often found in riverbeds or as nodules within limestone outcrops.
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