Agate (specifically Botswana or Lake Superior style chalcedony)

Physical Properties

Hardness: 6.5-7 (Mohs); Color: translucent brown to tan; Luster: waxy to vitreous; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: none; Conchoidal fracture; Specific gravity: 2.58-2.64

Formation & Geological History

Formed as a secondary deposit in volcanic rock cavities (vesicles) from silica-rich groundwater. Over millions of years, the silica precipitates into concentric layers or microcrystalline masses.

Uses & Applications

Used primarily in lapidary arts for jewelry, beads, and cabochons. Historically used for mortar and pestles due to its hardness and resistance to chemicals.

Geological Facts

Agates often feature 'eyes' or banding that is only visible when sliced or polished. They are a variety of chalcedony and are found inside geodes or as nodules in riverbeds.

Field Identification & Locations

Identify by its translucent quality and waxy surface when wet. Unlike common river stones, agates allow light to pass through the edges. Frequently found in glacial till, river beds, and gravel pits.

Identified on: 4/19/2026

Mode: Standard