sedimentary
Chert (Basanite / Jasper)
Microcrystalline quartz (SiO2)
Hardness: 6.5-7 (Mohs); Color: Dark olive green to black; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Dark olive green to black
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Dark olive green to black; 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 or chemical precipitation in sedimentary environments. Common in marine deposits where silica nodules grow within limestone or chalk beds.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; currently used for decorative stones, landscaping, and lapidary work.
Geological facts
Dark, fine-grained varieties of chert like this one are often referred to as 'touchstones' because they were historically used to test the purity of gold and silver by the color of the streak left behind.
Field identification & locations
Identify in the field by its smooth, waxy texture and distinctive conchoidal (shell-like) fracture. Commonly found in riverbeds, glacial tills, or coastal areas as rounded pebbles.
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