sedimentary
Chert with Quartz Vein
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale. Color: Brownish-tan to yellowish-brown with white quartz inclusions. Luster: Waxy to dull for chert, vitreous for quartz. Crystal structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None, exhibits conchoidal fracture. Specific Gravity: ~2.65.
- Hardness
- 7 on Mohs scale
- Color
- Brownish-tan to yellowish-brown with white quartz inclusions
- Luster
- Waxy to dull for chert, vitreous for quartz
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Physical properties
Hardness: 7 on Mohs scale. Color: Brownish-tan to yellowish-brown with white quartz inclusions. Luster: Waxy to dull for chert, vitreous for quartz. Crystal structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None, exhibits conchoidal fracture. Specific Gravity: ~2.65.
Formation & geological history
Chert forms through the accumulation of siliceous organism remains (like radiolarians or diatoms) or precipitation of silica on ocean or lake beds. Subsequent tectonic activity led to fracturing and fluid flow, depositing milky quartz veins inside the rock before weathering rounded it into a pebble.
Uses & applications
Historically used for stone tools and flintknapping due to its sharp conchoidal fracture. Modern uses include decorative landscaping gravel, stream bed fills, and lapidary tumbling for hobbyists.
Geological facts
Chert pebbles with white veins are often referred to as 'wishing stones' in folklore, where tradition holds that tracing a complete vein around the rock grants a wish.
Field identification & locations
Identify in the field by its hardness (scratches glass), smooth waxy feel, conchoidal fracturing, and distinctive white quartz cross-cutting veins. Extremely common along riverbeds, gravel bars, and ocean beaches worldwide.
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