There is a fabulous place in Utah called Topaz Mountain. Six or seven million years ago, a volcano erupted and formed a huge rhyolite deposit. As the rock formed, topaz (a semi-precious gem) also grew. We had a ton of fun with the kids and their aunts and uncles one day hunting for topaz. You can break open rhyolite to find topaz or you can spot it in washes and dried riverbeds...especially after a good storm! A big thanks to my mom for taking pictures after my phone died!
And here's a link for more information about Topaz Mountain: http://geology.utah.gov/utahgeo/rockmineral/collecting/topaz.htm