A Gem of a Different Color
On a beautiful October morning our small group eagerly piled into the SUV at Snow College. The time of year couldn’t have been more perfect; the weather was amazing. Our clan consisted of Snow students impassioned in the field of geology, and paleontology. We had our esteemed leader, Geologist and instructor Renee Faatz, to guide us. Sailing over the dirt roads, like a ship with the wind in her sheets, we first headed out to Topaz Mountain. The mountain is not much to look at with its gloomy, grey slopes and patchy scrub. But it seems that the most wonderful things come out of drab, dreary rock – and Topaz Mountain is certainly no exception. Besides bearing its namesake, Topaz Mountain’s rhyolite is also host to quartz, garnet, pseudobrookite, bixbyite, and the elusive red beryl. These are just a few of the treasures tucked away within the unassuming gray walls. The topaz, however, was the main reason for our being there.
The Thomas Range topaz formed from trapped volcanic gasses. Six to Seven million years ago, volcanic vents emerged along faults in the area. The thick, gaseous lava flow contained numerous bubbles called vugs. Inside the vugs, fluorine-bearing vapor sublimated from the lava. In the last stages of solidification, the trapped vapor cooled and formed beautiful topaz crystals.
The topaz at Topaz Mountain can be found in a small variety of colors. The colors range from a nice rich sherry, to light pink, to clear. The reason for this color palette has something to do with good ol’ wholesome sunshine. When the crystals are exposed to sunlight they tend to fade over time. I thought this was rather curious, so I decided to find out why. I rummaged through my field books, remembering that I own a copy about Topaz Mountain; the author and expert on this location, John Holfert, offers this explanation:
Unfortunately, the color of the Thomas Range topaz is not stable when crystals are left exposed to direct sunlight for extended periods of time. . . .The sherry color of the unexposed crystals is a direct result of exposure to naturally occurring ground radiation for millions of years, probably from trace amounts of uranium in the rhyolite. Radiation causes electrons to be displaced to a higher energy state giving the crystal a temporary color center. Exposure to direct sunlight excites the electrons causing them to return to their normal state, thereby eliminating the color center, resulting in a color shift from sherry to colorless. (4)
Holfert goes on to express that the rich sherry color can be restored if the crystal is exposed to, “strong radiation for a short period of time” (4). This makes sense because I also found out that this is precisely how most blue topaz are created. According to the Department of Geological Sciences at the University of Texas, “Most natural topaz is colorless or very pale blue; the dark blue color, so commonly seen today is produced by irradiation, usually followed by heating” (Topaz).
Holfert assures his readers that the color change takes about a week to ten days to take place. He also states, “Artificial light, including florescent and halogen light, does not appear to have any negative effect on the color stability of the topaz” (5). I found this to be a relief because I was trying to keep my topaz in eternal darkness to preserve their coloring. Now I can display them without any worries, as long as they stay out of direct sunlight.
After a very satisfying expedition to Topaz Mountain we were back in the SUV, being blown by the wind to our next grand adventure. Most of us found some very pretty topaz crystals. We all had a really great time.
Notice how the topaz in the foreground is a light pinkish color, while the topaz in the back is a deeper sherry hue.
In contrast, this topaz from Topaz Mountain is colorless. (Mike)
Holfert, John. A Field Guide to Topaz and Associated Minerals of the Thomas Range, Utah (Topaz Mountain) Volume 1. UT: HM Publishing, Dec. 1996. Print.
Mike. CSMS Geology Post. Colorado Springs Mineralogical Society. 5 June 2013, Web. 18 October 2014.
Topaz. Deptartment of Geological Sciences, University of Tx. 1998, Web. 18 October 2014.