sedimentary
Chert (Yellow variety / Jasper)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale. Color: Cream to yellow (often caused by goethite or iron oxides). Luster: Waxy to vitreous. Structure: Cryptocrystalline. Cleavage: None (exhibits prominent conchoidal fracture). Specific Gravity: 2.6–2.7.
- Hardness
- 6
- Color
- Cream to yellow (often caused by goethite or iron oxides)
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5–7 on Mohs scale. Color: Cream to yellow (often caused by goethite or iron oxides). Luster: Waxy to vitreous. Structure: Cryptocrystalline. Cleavage: None (exhibits prominent conchoidal fracture). Specific Gravity: 2.6–2.7.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation in marine or groundwater environments. It often forms as nodules within limestone or chalk beds.
Uses & applications
Used historically for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in jewelry (cabochons), as an abrasive, and sometimes in construction aggregate or landscaping.
Geological facts
Chert and its variety Jasper have been used by humans for over 2 million years. When struck against steel, it can produce sparks, though flint (a grey/black variety) is more famous for this. The bandings seen in specimens like this often reflect changes in water chemistry during formation.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fractures and its ability to scratch glass easily. It is found globally, notably in riverbeds and limestone outcrops. Collectors should look for unique patterns or 'desert varnish' on the exterior.
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