sedimentary
Chert with Chalcedony/Quartz Vug
Cryptocrystalline Silica (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, white, Luster: waxy to dull, Crystal structure: Microcrystalline/Trigonal, Cleavage: None (conchoidal fracture).
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, white, Luster: waxy to dull, Crystal structure: Microcrystalline/Trigonal, Cleavage: None (conchoidal fracture).
Formation & geological history
Formed primarily through the chemical precipitation of silica from groundwater filling cavities (vugs) in limestone or through the replacement of calcium carbonate. This piece shows banded chalcedony layers and a druzy quartz center.
Uses & applications
Historically used for stone tools (arrowheads); currently used for decorative purposes, lapidary tumbling, and as a collector's specimen.
Geological facts
Chert is essentially the same mineral as jasper or flint, with names often changing based on color and transparency. This specimen shows a transition from common chert to crystallized quartz.
Field identification & locations
Identify by its hardness (it will scratch glass) and waxy luster on broken surfaces. Often found as nodules in sedimentary limestone beds or in gravel deposits.
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