Thomas Mountains of Utah - Volcanic Fields for Hundreds of Miles

Thomas Mountains, Utah

This summer when I did my dinosaur expedition to Utah, we took some time to go to the Thomas Mountains in eastern Utah.  We actually went there last year as well.  This place is amazing!  The geology of the region is very complex, with multiple events occurring in the same place over a span of hundreds of millions of years.  The Thomas Mountains are a volcanic mountain range that formed about 50 million years ago to as recently as 1 million years ago, near the time of the end of the formation of the Rocky Mountains.  Numerous volcanoes litter the region and most of them are calderas, giant volcanoes that erupted very violently.  Because of the volcanic activity in this area, gemstones are very common throughout the region. 

Probably the best known caldera is Topaz Mountain, appropriately named because natural amber-colored topaz is found here, along with red beryl, amethyst, garnet, bixbyite, opal and hematite.  The minerals are found in cavities in the rhyolite ash that was deposited when the volcano erupted.  It can be a lot of work to break open the rhyolite to find the cavities with the precious minerals inside.    In the picture, you can see the crater of the caldera.  The area of Eureka is also famous for many metal deposits, including gold and silver.

Topaz Mountain near Eureka, Utah

To the southeast of Topaz Mountain, near the town of Nephi, precious calcite onyx can be found.  The popular flowering tube onyx is from this area as well as yellow, red and white banded sardonyx, which will be available on Ruby Mountain5 Rocks soon.  The onyx forms when geothermal fluids from the volcanoes flowed through calcium-rich rocks in the mountains, depositing bands of colorful silica.

Flowering Tube Onyx

 

Sardonyx

Available at Clidastes Stones

To the west of Eureka is the town of Birdseye, where birdseye rhyolite is found (Utah Birdseye Rhyolite will soon be available at Ruby Mountain5 Rocks, currently you can find Mexican Birds Eye Rhyolite)

Birdseye Rhyolite, Utah

Available at Clidastes Stones

South of Delta is another kind of volcano, Sunstone Knoll is a locality that is andesite lava.  In the cavities in the andesite, you can find small yellow sunstone crystals.  Sunstone is a gem variety of the mineral feldspar, the same class of minerals that includes moonstone and labradorite.

Sunstone knoll is a cinder cone.  Cinder cones are smaller volcanoes that are less explosive and produce more lava than ash.

Sunstone Knoll.  Image from millardcounty.com

Sunstones collected from Sunstone Knoll

My most favorite gemstone from this area is the bacon opal.  Bacon opal is formed when geothermal fluids flow through rhyolite layers and deposit colorful bands of silica rich in iron minerals.  Every piece of this material is unique.  You can find specimens of bacon opal at Ruby Mountain5 Rocks, Bacon Opal.

 

One of the most famous and sought after gemstones from Thomas Mountains is Tiffany Stone.  Tiffany stone is becoming very rare because the locality has not been open to collecting for quite a long time and the only stock available was collected over 30 years ago.  Tiffany stone is opalized beryllium-rich fluorite and bertrandite.  This stone has a cracked marbling of creamy white and purple.  You can get your own Tiffany Stone at Ruby Mountain5 Rocks.

 

Utah is riddled with collecting localities for rock hounds of all types.  Some localities can be found here and here.

By Dr. Janet Bertog, Professor of Geology
Owner of Ruby Mountain5Rocks
Owner of Clidastes Stones