Sweetwater Agate, Agate Flats, Wyoming

Contributed by: Chris Clemens
minID: 80K-WUK
Date: Apr 25th, 2026
Locality: Fremont County, Wyoming, USA (See on Mindat)
Size: 32 x 26 x 57 mm
Weight: 49 g

Description:
Translucent, bluish/grey, river-worn, nodule of chalcedony with numerous radiating inclusions of manganese oxide. Found in the Agate Flats area, north of the Sweetwater River, Fremont County, Wyoming. Named after the Sweetwater River, Sweetwater agate shows a bright green, fluorescent response under shortwave UV due to the inclusion of trace amounts of uranium (in the form of the uranyl ion), which also causes this material to emit a low level of radioactivity (around 90 cpm). This specimen shows a less-bright green response under midwave UV, and a dim green response under longwave UV. Sweetwater agate is also a popular lapidary material.

Originally posted by Chris Clemens on Nature's Rainbows in 2016.

Fluorescence under shortwave UV light.

Fluorescence under midwave UV light.

Fluorescence under longwave UV light.

Normal light.

Summary of luminescence responses:

Chalcedony (Mindat) (RRUFF)

• Fluorescence under Shortwave (254nm Lamp/Mercury) UV light: Green
• Fluorescence under Midwave (305nm Lamp/Mercury) UV light: Green
• Fluorescence under Longwave (365nm LED) UV light: Green