The Thomas Range, located near the town of Delta in the West Desert area of Utah, is a small mountain range that formed approximately 7 million years ago, and is composed of a series of rhyolitic lava flows. The siliceous rhyolite provides a source of silica that can form crusts of glassy hyalite opal (Opal-AN) on the surface of the rhyolite, and is often brightly fluorescent due to uranium content.
This specimen is an excellent example of hyalite opal on rhyolite matrix from the Thomas Range, and consists of a transparent crust of bubbly hyalite on a light brown/grey colored rhyolite matrix. The hyalite opal fluoresces bright green under short wave UV and dimmer green under long wave UV. The size of this specimen is 68 x 63 x 29 mm, and it weighs 77 grams. Due to its uranium content, this specimen emits a low level of radioactivity at approximately 200 cpm (at 1 cm).
The following photographs show a full view of the specimen under short and long wave UV, and visible light:
Mouse over the scrolling images for UV wavelength information.
The following macro photographs show close-up detail of the layer of hyalite opal under short and long wave UV, and visible light. Field of view is approximately 32 mm.
Mouse over the scrolling images for UV wavelength information.