Youngite, from the Youngite Caves, Guernsey, Wyoming

Contributed by: Chris Clemens
minID: 7KC-XUP
Date: Apr 25th, 2026
Locality: Guernsey, Platte County, Wyoming, USA (See on Mindat)
Size: 8.9 x 5.1 x 10.2 cm
Weight: 269 g

Description:
Fluorescent youngite specimen from Guernsey, Wyoming. Youngite is an agatized brecciated jasper with a coating of grayish translucent druzy quartz. Found in only one location, the youngite caves near Guernsey, Platte County, Wyoming. The fluorescent response of youngite under UV is highly variable, with some specimens showing spectacular fluorescence, and others being only dimly fluorescent. This specimen is about the best I’ve seen. Under SW UV the drusy quartz fluoresces green, and the jasper center fluoresces a blue/grey color with deep blue phosphorescence. Under midwave UV this piece looks like an entirely different specimen, with the quartz druse showing a beautiful pastel blue and green fluorescence, and the jasper fluorescing a cream/brown color. The jasper is also dimly phosphorescent after midwave UV. The longwave response is more subtle, with the jasper responding a brighter cream/brown and the quartz druse fluorescing darker green.

The Youngite caves are located on private property and have been off-limits to collectors for years.

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