sedimentary
Vuggy Chert / Silicified Limestone
Silicon Dioxide (SiO2) with Iron Oxide (Fe2O3) inclusions
Hardness: 6.5-7 on Mohs scale; Color: Rusty brown, reddish-orange, tan, and beige; Luster: Dull to earthy, sub-vitreous in places; Structure: Microcrystalline with irregular vugs and cavities; Cleavage: None (conchoidal to uneven fracture).
- Hardness
- 6
- Color
- Rusty brown, reddish-orange, tan, and beige
- Luster
- Dull to earthy, sub-vitreous in places
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Rusty brown, reddish-orange, tan, and beige; Luster: Dull to earthy, sub-vitreous in places; Structure: Microcrystalline with irregular vugs and cavities; Cleavage: None (conchoidal to uneven fracture).
Formation & geological history
Formed by secondary replacement of host limestone or dolomite by silica-rich fluids. Dissolution of original carbonates creates the characteristic vuggy or pitted texture, while leaching of iron minerals imparts reddish-orange staining.
Uses & applications
Used historically by Indigenous populations for stone tools and knapping due to its hardness; used today as raw material for landscaping, rock tumbling, or as a decorative geological sample.
Geological facts
Vuggy chert specimens often contain fossil molds or tiny quartz crystal linings inside their cavities. The reddish-brown coloration is a result of iron oxide weathering, often hematite or goethite.
Field identification & locations
Identified by its microcrystalline texture, hardness (scratches steel), irregular porous cavities, and iron-stained rusty surfaces. Commonly found in limestone karst terrains and stream beds in regions like the American Midwest (e.g., Ozarks).
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