Agate (Moss or Lake Superior type)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Variegated brown, orange, black, and translucent grey; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture).

Formation & Geological History

Formed in cavities (vesicles) of volcanic lava flows, such as those that occurred over 1 billion years ago in the Midcontinent Rift System. Ground water rich in silica seeped into bubbles and hardened over time, creating layers.

Uses & Applications

Used primarily for lapidary work (jewelry, cabochons), decor, and as popular collectibles for rockhounds.

Geological Facts

Agates are known for their banded patterns and translucency. The Lake Superior Agate is the state gem of Minnesota and is prized for its iron-rich red and orange colors.

Field Identification & Locations

Look for a waxy, 'plastic-like' texture and pitted surfaces. Translucency when held up to a bright light is a primary identification tool in the field. Found in gravel pits and along shorelines.

Identified on: 4/16/2026

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