Rare Green Fluorescent Fluorite - Fourmile Creek, Colorado

Contributed by: Chris Clemens
Date: May 1st, 2026
Locality: Fourmile Creek Area, Fremont County, Colorado, USA (See on Mindat)
Size: 8 x 6 x 10 cm
Weight: 376 g

Description:
Fluorite is a mineral that occurs in many different colors and habits and is often fluorescent under UV. Most commonly, fluorite shows a deep, intense blue fluorescence under longwave UV, but some varieties are known to respond with different colors under UV. Green is not a common color of fluorescence for fluorite, but an unusual purple-grey colored botryoidal form of this mineral, found in the Fourmile Creek area, near Canon City, Fremont County, Colorado, shows a rare green response under shortwave UV. Here is an example of such a specimen.

In addition to its unusual fluorescent response under UV, the fluorite from Fourmile Creek exhibits an uncommon botryoidal habit and purple-grey color when viewed under visible light. The fluorite has formed on the surface of an unidentified red/brown non-fluorescent matrix.

As seen in the closeup images of a broken surface showing a cross section of one of the hemispherical regions (seen at lower left in the photographs of the full specimen), the fluorescence is not merely a surface phenomenon but occurs throughout the interior of the fluorite.

In his book, "Fluorescence- Gems and Minerals Under Ultraviolet Light (1994)", Manuel Robbins speculates that the mechanism of green fluorescence in these Fourmile Creek fluorites is likely due to uranyl-activation. In fact, this specimen is radioactive, having a measured emission of approximately 200 cpm. This finding is indicative of the presence of uranium in this specimen, suggesting that Robbin's hypothesis for uranyl-activation is correct.

Originally posted by Chris Clemens on Nature's Rainbows in 2017.