Gem Profile- Rhyolite

by Judy Ellis

When I was a child, my family took a vacation to Yellowstone every year where I ended up with more pictures of the rocks than of the geysers. One year we took a trip to Crater Lake in Oregon and I was amazed at the Lake and the absolute power of the volcano that created it. Later on, I watched in fascination as Mt. St. Helens exploded causing the same destruction as a 5-mega-ton bomb. At this point I thought that all volcanoes acted the same; they exploded causing major damage. It wasn't until after I visited Hawaii that I really began to research and discover the differences not only in volcanoes, but in the types of magma and lava that create them. One of these types of lava is the subject of this profile; Rhyolite.

What is Rhyolite?

Rhyolite gets its name from the Greek word Rhyax, meaning stream.

This describes how the lava flows, forming visible bands in the rock.

You may also see Rhyolite called Rainforest Jasper, although this is a misnomer. Jasper is a metamorphic silicate rock with small crystals and high silicon content.

Rainforest Jasper looks similar to Rhyolite and contains almost the same minerals, which likely explains the name. This Rhyolite variety is mainly found in Australia. It is a lime-green stone with purple and clear quartz crystals suspended within the matrix.

Rhyolite is an extrusive igneous rock that shares the same chemical composition as Granite. "Extrusive" means the rock is pushed out from the ground, and "igneous" means it originates from magma.

Rhyolite is also highly viscous, meaning it does not flow easily. It tends to cool quickly and can "plug" a volcano, causing a very violent eruption as gasses build up pressure.

A 7-pound chunk of "Rainforest Jasper" rough, similar in looks to Ocean Jasper. Private collection, Dale Armstrong

About Rhyolite

When Rhyolite erupts, it can form several different types of stone. For example, pumice, which is used for cosmetic or decorative purposes, is Rhyolite with many trapped air bubbles.

The Maori of New Zealand used a form of Rhyolite, similar to obsidian, for their weapons. This type cooled so quickly that gas bubbles could not form.

Rhyolite that cools at different rates can create varied textures. It may form a light-colored stone with very small grains or a more porphyritic texture.

The porphyritic texture features larger grains suspended in a "glassy" matrix. This is the type of Rhyolite used in jewelry making.

The jewelry-grade Rhyolite may contain visible crystals of quartz, mica, chalcedony, feldspar, and other minerals that form as the stone cools.

As a general rule, the longer a rock takes to cool, the larger the crystals that form within it. Rhyolite lava often cools very quickly, which is why the stone used for jewelry, with its larger crystals, is fairly rare.

Polished Rhyolite cabochon at a gem show in Tucson. See the pretty windows in the stone?

Finding Rhyolite: Road Trip!

This week, we are taking a special look at the town of Rhyolite, Nevada.

Although it is now a ghost town, Rhyolite was a booming gold mine in the early 1900s.

Rhyolite is located near Beatty, Nevada, on the way to Death Valley, California.

As you may have guessed, the town was named after the plentiful Rhyolite stone in the area. It sat in the middle of the Bullfrog Mining District.

For about ten years, Rhyolite was the largest town in the entire region.

Camping facilities are available in Beatty. The ghost town is open for exploration and rock-hounding in the nearby Rhyolite hills.

Who knows, yo

Aren't these Rhyolite beads beautiful? The good news, they are available on Wirejewelry.com

If you are going rock-hounding, remember that Rhyolite is a fairly common rock found on every continent.

It is a light-colored stone with few visible crystals. It can also appear as pumice or obsidian and shares the same chemical formula as granite.

Rhyolite ranks 6 to 7 on the Mohs scale. Historically, it has been used in ornamental carvings and jewelry.

Even though it is common, Rhyolite is still a bit of a mystery, especially regarding the crystals that form within its matrix.

Some crystals form at different temperatures than the cooling stone, and others form at great depths within the earth. This makes Rhyolite a very special and interesting stone indeed.

Gem Profile- Rhyolite

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