sedimentary

Chert (Common Opal/Agatized Rock)

Microcrystalline Quartz (SiO2) with potential Chalcedony/Opaline silica

AI-generated identification · may be incorrect

Hardness: 6.5-7 Mohs; Color: Tan, white, and reddish-orange; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Hardness: 6
Color: Tan, white, and reddish-orange
Luster: Waxy to dull

Identified July 9, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Tan, white, and reddish-orange; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Formation & geological history

Formed via the accumulation of silica-rich fluids in sedimentary environments or as a replacement of organic material and limestone. Commonly found in nodules or bedded deposits within limestone or dolomite.

Uses & applications

Used historically for stone tools due to its conchoidal fracture; currently used as landscaping stone, in road construction, and as a collector's specimen if aesthetically pleasing.

Geological facts

This specimen shows characteristic waxy luster and conchoidal fracturing common in silica minerals. The reddish tints are likely caused by iron oxide (hematite) inclusions.

Field identification & locations

Identify in the field by its hardness (it will scratch glass) and its sharp, shell-like fracture edges. It is often found in riverbeds or eroding out of limestone cliffs.