Banded Agate (Lake Superior Agate type)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Red, orange, tan, and white concentric bands; Luster: Waxy to vitreous; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None/Conchoidal fracture; Specific Gravity: 2.58-2.64.

Formation & Geological History

Formed in gas vesicles (bubbles) within basaltic lava flows approximately 1.1 billion years ago during the Midcontinent Rift. Iron-rich silica-bearing groundwater later filled these cavities, depositing layers of microcrystalline quartz.

Uses & Applications

Commonly used for lapidary work, jewelry (cabochons), collection, and decorative items. In industry, due to its hardness and chemical resistance, agate has been used to make mortars and pestles.

Geological Facts

This specimen shows the characteristic 'fortification' banding patterns. It is likely a Lake Superior Agate, which is the state gem of Minnesota. The red and orange hues are caused by varying amounts of oxidized iron (hematite and limonite).

Field Identification & Locations

Identify in the field by looking for translucent, banded patterns and a waxy texture on weathered rinds. They are frequently found in glacial till and gravel pits across the Upper Midwest USA. Use a flashlight to check for translucency.

Identified on: 4/22/2026

Mode: Standard