sedimentary
Chert (Yellow Jasper)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Yellow, mustard, ochre; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Yellow, mustard, ochre
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Yellow, mustard, ochre; 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 microfossils (like radiolaria or diatoms) or chemical precipitation from silica-rich groundwater within sedimentary beds. The yellow color typically comes from iron oxide (goethite) inclusions.
Uses & applications
Used historically for stone tools due to its sharp edges when fractured; modernly used for lapidary work, jewelry (cabochons), and as aquarium stones.
Geological facts
Chert and jasper have been used by humans for over a million years to create spearheads and arrowheads because they fracture predictably with very sharp 'razor' edges.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and its shell-like (conchoidal) fracture pattern. It is commonly found in limestone and chalk deposits globally.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Brecciated Chert (Microcrystalline Silica)
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