Banded Agate (specifically Sardonyx or Carnelian Agate)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Red, orange, white, and brown concentric bands; Luster: Waxy to vitreous; Crystal Structure: Trigonal (Cryptocrystalline); Cleavage: None; Specific Gravity: 2.58-2.64

Formation & Geological History

Formed through the deposition of silica from groundwater into cavities in igneous or sedimentary rocks. The banding represents different stages of mineral deposition over time. Most high-quality banded agates date from the Cenozoic era.

Uses & Applications

Used primarily in lapidary arts, jewelry, decorative carvings (like this polished tower), and historically for cameos, seals, and precision tool bearings.

Geological Facts

Agate has been used as a gemstone for over 3,000 years. The distinct 'eyes' or circular bands are often referred to as 'orbicular' patterns. Redder varieties like this are often heat-treated to enhance their natural iron-oxide colors.

Field Identification & Locations

Identify in the field by its distinctive concentric banding, waxy texture when fractured, and its ability to scratch glass. Commonly found in volcanic regions of Brazil, Madagascar, and the Lake Superior region of the US.

Identified on: 4/29/2026

Mode: Standard