Agate

Physical Properties

Hardness: 6.5-7 (Mohs scale), Color: Varies Widely (this specimen is dyed blue), Luster: Vitreous to waxy, Crystal Structure: Trigonal/Microcrystalline, Cleavage: None, Specific Gravity: 2.58-2.64

Formation & Geological History

Formed in cavities of volcanic rocks (lavas) or metamorphic rocks by the deposition of silica from groundwater. The layers are formed by different mineral inclusions or changes in iron content over time.

Uses & Applications

Used for ornamental purposes, jewelry (cabochons), interior decor, bookends, and industrial use in precision mortars and pestles due to its hardness.

Geological Facts

This specific piece is a 'thunderegg' or geode slice that has been artificially dyed blue. Natural blue agate exists (Blue Lace Agate), but this vibrant, uniform indigo is characteristic of a dye process used to enhance the appearance of common grey agate.

Field Identification & Locations

Identify by the distinctive concentric banding. In the field, look for nodules with a waxy luster on fractured surfaces. They are common in volcanic regions like southern Brazil, Uruguay, and the Lake Superior region of the USA.

Identified on: 4/19/2026

Mode: Standard