sedimentary
Chert (Yellow Jasper varieties)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale; Color: Yellow, mustard, ochre; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; 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: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
Formation & geological history
Formed through the accumulation of silica-rich microfossils or chemical precipitation from groundwater within sedimentary layers. This specific yellow color is typically due to iron oxide (limonite) inclusions.
Uses & applications
Historically used for stone tools (flintknapping); currently used in jewelry (cabochons), lapidary art, and as an ornamental stone in landscaping.
Geological facts
Chert is one of the most durable sedimentary rocks. Yellow varieties like the one shown are often specifically called 'Yellow Jasper' when they are opaque and take a high polish.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture, ability to scratch glass, and waxy luster on fresh surfaces. It is found globally in limestone and chalk deposits.
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