(Field trip article written by Rick Dalrymple about Topaz Mountain.)
Topaz Mountain is on the southern end of the Thomas Range inside Juab County. The Thomas Range is a rhyolitic flow. This means that around 6-8 million years ago, lava flowed on several occasions. The lava was rhyolitic. Rhyolite is the same as granite but extrusive (formed on the outside of the crust.) It is within this rhyolite that we are hunting.
This is a list of minerals from the Thomas Range: bixbyite, cassiterite, chrysocolla, durangite, fluorite, garnets, hematite, mimetite, opal, pseudobrookite red beryl, topaz, weeksite, and zircon.
I'm sure there are others, but this gives a good indication of what you can find.
Some of these minerals are microscopic, and we won't bother looking for them. The ones we will look for are bixbyite, fluorite, opal, red beryl and topaz. The reason we will look for the fluorite and opal is because they indicate a good digging area for the topaz.
Be patient! Look for bushes, grass and trees growing out of cracks. These cracks can be layers, seams, cavities or some kind of structural feature in the rhyolite. These features occur where the topaz is located. Use a screwdriver to probe the seams to test for soft spots that may lead to a pocket. Probe in all directions. Most cavities go unnoticed.
Fluorite, opal, amethyst and hematite are all very good indicators of a topaz seam or cavity. However, not every seam that has fluorite, opal, quartz or hematite will have topaz.
Here is a description of some of the minerals:
Topaz. [Al2SiO4(FOH)2] Luster: glassy Hardness: 8 Cleavage:perfect basal Color: clear to dark cinnamon colored. If the colored topaz is left in the sunlight, the color is washed out by the ultraviolet light. So if you find a good colored one, wrap it up.
Bixbyite. [(MnFe)2O3] Luster: metallic black Hardness: 6-6.5 Cleavage: poor Color: black The Thomas Range is type locality for bixbyite. This means it was first discovered here. It was discovered by Maynard Bixby (who it was named after) in the late 1890's. It forms into small black cubes, often on topaz.
Red beryl. [Be2All2Si6Ol8] Luster: glassy Hardness: 7.5+ Cleavage: poor Color: rose to violet pink Red beryl is the most rare and valuable of all gemstones. It is found only in two places in the world. Both are in Utah. The red beryl found at the Thomas Range is not the gem quality material that is so valuable. The valuable red beryl is from the Wah Wah Mountains further south. It is privately owned and no collecting is allowed. The red beryl form the Thomas range forms in small discs up to 1/4" round and 1/16" thick. In the gem community it is known as bixbite, after Maynard Bixby who discovered it in the Thomas Range at the same location that bixbyite was first found. This makes the Thomas Range the type locality for red beryl.
Other things to collect in the area: Apache tears (obsidian), bertrandite nodules, blue agate, butterscotch jasper, frothy rhyolite, geodes, snowflake obsidian.
The most important thing to remember about the collecting at Topaz Mountains is that it doesn't like to give up its treasuresóbe patient!!!