sedimentary
Chert (or Agate/Quartzite)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: brown, tan, and gray with translucent patches, Luster: waxy to vitreous when wet, Crystal Structure: hexagonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs), Color: brown, tan, and gray with translucent patches, Luster: waxy to vitreous when wet, Crystal Structure: hexagonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed via the accumulation of silica-rich microfossils or chemical precipitation in marine environments, often under high pressure within limestone or marl. Frequently found as weathered river/beach pebbles.
Uses & applications
Historically used for making stone tools; currently used as decorative landscaping stone, in lapidary work (tumbling), and sometimes as industrial abrasives.
Geological facts
Chert has been used by humans for over 2 million years to create sharp-edged tools and sparks for fire. When it is translucent and banded, it is referred to as Agate.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth waxy feel, and characteristic shell-like (conchoidal) fractures on broken edges. Common in riverbeds and gravel pits.
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