|North America||Thomas Mountains, Juab County, Utah, USA||1 ¼" x 1" x 1"|
This specimen is from a well-known Utah locality: Topaz Mountain in the Thomas Range, west of the town of Delta. The mineral occurs in rhyolite, a volcanic rock with about the same composition as granite (which is not volcanic). Over time the rock slowly erodes, exposing topaz crystals, which are eventually freed from the rock and washed downhill into the (usually) dry stream beds below. Many years ago the road from the highway north of Delta out to Topaz Mountain was unpaved, so fewer people visited the spot, and it was easy to find good topaz crystals in the sand of those dry stream beds. Paving of the road resulted in increased "rockhounding," and it has become more difficult to find good crystals in the stream beds. This is because the rate of crystal collection greatly exceeds the rate of erosion.
The "collectible" crystals like this come from cavities in the rhyolite. When the crystals are exposed by careful hammering on the rock, they are "sherry" colored, a light pinkish brown. With exposure to sunlight (for example, while lying in a dry stream bed for years), the color is bleached and they become colorless, like this specimen. That is because the color is caused by a color center that is unstable with prolonged exposure to sunlight. To see the original color, look at specimen #60, also from this locality.
Clusters of topaz crystals, or those with their bases still in the rock, are unusual from Topaz Mountain. This is because of the perfect and easy cleavage (see Learn More for specimen #18 for an explanation of cleavage in topaz). The cleavage planes are perpendicular to the lengths of the crystals, and any small bumps will cause them to break away from the rock or other crystals with which they may be intergrown.