sedimentary

Chert / Jasper / Agate (Chalcedony Group)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Multi-colored (red, green, white, gray, brown); Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Multi-colored (red, green, white, gray, brown)
Luster: Waxy to vitreous

Identified February 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the precipitation of silica-rich fluids in sedimentary environments (often replacing organic matter or filling voids in limestone) or as secondary deposits in volcanic rocks. Can range from Precambrian to Cenozoic in age.

Uses & applications

Historically used for stone tools (arrowheads/knives) due to sharp edges; currently used for lapidation, jewelry (cabochons), aquarium decor, and aggregate in construction.

Geological facts

These specimens show classic conchoidal (shell-like) fracturing, which is why ancient civilizations prized them for making tools. The red varieties (Jasper) get their color from iron inclusions, while green may come from chlorite or iron silicates.

Field identification & locations

Identify in the field by its hardness (will scratch glass), waxy feel, and smooth curved break patterns. Found globally, especially in riverbeds, desert pavements, and exposed sedimentary layers. Popular for rock tumbling.

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