The topaz here comes out amber and then fades. Crystals from Topaz Mountain are naturally amber coloured, and sunlight turns them colourless, which means the stone in your hand is not the stone you found unless you keep it dark. Utah made topaz its state gem.
The rock that holds them is the Topaz Mountain Rhyolite, erupted six to seven million years ago from vents along faults in the area. The crystals grew into cavities inside it as it cooled, which sets up the two ways of collecting. Single colourless crystals, usually under an inch, wash out into the gullies and can simply be picked up. The bigger amber ones, and the clusters, come from breaking open the white rhyolite to find the cavities. The same rock yields red beryl, amethyst, garnet, opal, hematite, pseudobrookite and bixbyite.
The BLM manages this as the Topaz Mountain Rockhound Recreation Area and describes the surrounding hills as some of the most productive and diverse rockhound country in the west. There are no facilities of any kind, but dispersed camping is allowed throughout, and the area sits about 55 miles west of Delta. Two other well-known sites are close by: the trilobite beds at Antelope Springs and the geode beds near Dugway.
One caution about the name. The USGS deposit record called Topaz Mountain is not the collecting area. It carries alternate names including Spor Mountain and the Topaz Properties of Brush-Wellman, its commodities are beryllium and uranium, and its ore mineral is bertrandite. That is a large open-pit operation discovered in 1959 and in production since 1969, working a tabular hydrothermal body controlled by faulting. Same name on the map, entirely different ground and rules.
Topaz (Utah state gem), red beryl
Other places on the map within a couple of hours.
