Agate (Nodule/Chert)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, white, and translucent with banding; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline/cryptocrystalline quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64.

Formation & Geological History

Formed through the deposition of silica from groundwater in the cavities of igneous rocks or as nodules in sedimentary limestone or chalk. Many specimens date back to the Cretaceous or Tertiary periods.

Uses & Applications

Used primarily for jewelry (cabochons), lapidary arts, ornamental carving, and architectural accents. Historically used for making pestles and mortars due to its hardness.

Geological Facts

The name 'Agate' comes from the Achates River in Sicily. It is one of the oldest materials used in human history for adornment and tools.

Field Identification & Locations

Identify by its waxy appearance, translucency at the edges, and characteristic 'bubbly' or banded patterns. It is commonly found in river beds, beaches, and volcanic soil in regions like the Lake Superior area, Brazil, and India.

Identified on: 5/2/2026

Mode: Standard