sedimentary
Chert with Quartz Vein
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey to black matrix with white vein, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark grey to black matrix with white vein, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed primarily as a chemical precipitate or through the accumulation of siliceous biological remains (like radiolarians) in deep-sea environments. The white vein formed later via hydrothermal activity where quartz-rich fluids filled a fracture.
Uses & applications
Historically used for stone tools; currently used in construction aggregate or as decorative landscape stone. Dark chert with prominent veining is sometimes polished for lapidary use.
Geological facts
Chert is so hard it can strike sparks against steel, making it a primitive fire-starting material. The dark color often comes from organic matter trapped within the silica structure.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture and high hardness (it will scratch glass). Commonly found in limestone or chalk deposits globally.
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