Chert (Jasper/Opaline variety)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Dull greenish-yellow to tan; Luster: Waxy to vitreous; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Formation & Geological History

Formed from the accumulation of silica-rich microfossils or via chemical precipitation from silica-rich fluids in sedimentary basins or volcanic environments. Often associated with Cenozoic sedimentary layers.

Uses & Applications

Historically used for stone tools due to its sharp edges; currently used for lapidary work, landscaping, and as a collector's specimen.

Geological Facts

Chert is so hard that it can scratch glass and sparks when struck against steel. This specific waxy, greenish-tan appearance is often called common opal or jasper-chert depending on water content.

Field Identification & Locations

Identify in the field by its smooth, shell-like fracture (conchoidal) and extreme hardness. Commonly found in riverbeds, desert pavements, and exposed sedimentary outcrops across the Western United States.

Identified on: 5/1/2026

Mode: Standard