sedimentary
Chert (variations include Chalcedony and Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, white, and reddish-brown, Luster: waxy to dull, Crystal structure: Hexagonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, white, and reddish-brown, Luster: waxy to dull, Crystal structure: Hexagonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed through the precipitation of silica-rich fluids within sedimentary environments, often deep sea or limestone beds. This specimen likely formed as a replacement nodule in limestone or chalk during various geological periods ranging from Precambrian to Cenozoic.
Uses & applications
Historically used for making stone tools and arrowheads; currently used in construction as road base, in landscaping, and as a raw material for ceramics and glass when very pure.
Geological facts
Chert was one of the first minerals used by early humans for tools because its conchoidal fracture produces extremely sharp edges, similar to glass. It is often found as nodules inside limestone beds.
Field identification & locations
Identify in the field by its hardness (will easily scratch glass) and its smooth, shell-like fracture surfaces. It can be found in limestone outcrops, riverbeds, and coastal cliffs. Collect for lapidary work or flint knapping.
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