sedimentary

Chert (Chalcedony/Agate variety)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 Mohs; Color: Tan, white, and reddish-brown; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65

Hardness: 6
Color: Tan, white, and reddish-brown
Luster: Waxy to vitreous

Identified July 9, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Tan, white, and reddish-brown; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65

Formation & geological history

Formed primarily by the accumulation of siliceous remains of marine organisms or via precipitation of silica-rich ground fluids in limestone or volcanic cavities. Often occurs as nodules or beds in sedimentary layers.

Uses & applications

Historically used for stone tools (arrowheads); currently used as an abrasive, in decorative landscaping, and for lapidary work like tumbling and cabochon cutting.

Geological facts

Chert is so durable that it often survives long after its surrounding host rock has weathered away. It was one of the most important minerals for ancient humans due to its predictable conchoidal fracturing.

Field identification & locations

Identify by its waxy appearance, extreme hardness (scratches glass), and smooth, shell-like fracture surfaces. Often found in riverbeds, gravel pits, and limestone outcroppings.