Hunting for Topaz
This past May, I had the opportunity to do what every good geologist loves to do - dig around in the dirt. Specifically, geologists enjoy a good hunt for cool rock specimens and crystals, scrubbing them clean of any clay and placing them on their mantle to show off to the rest of their geo-friends. With wine and beer, of course.
I'm no different. I get very excited on long drives whenever I see a road cut. It's almost comical how my husband has to stop me from swerving on the road to point something out, or if I'm the passenger, keep me from rolling down the window and sticking my head out like an excited puppy to get a better look.
Gemology a little similar, except perhaps many gemologists would rather not hunt in the dirt, instead expertly grade their favorite minerals under a microscope or admire a cut stone ready to be set into a ring. Still, an occasional jaunt out into the real world can't hurt, and I was thrilled when the GIA Alumni Association of Houston offered a trip to hunt for topaz gemstones out in Llano, Texas.
Topaz is a neat little mineral. It's part of the silicate family, which makes it very hard and somewhat common as gemstones go. Even for the silicate family though, topaz is very dense and hard (Mohs hardness of 8), and depending on its impurities, it can come in an array of natural colors, including blue, yellow, golden, purple, pink and brown. The shape of topaz is its finest form is orthorhombic, which is a fancy geometric way of saying the sides of the crystal meet at right angles, but the lengths of those sides are uneven. In other words, it looks like a rectangle. When most people think of topaz, they either imagine the bold golden hues of the Brazilian imperial topaz, or they think of the December birthstone of blue topaz.
Turns out that blue topaz is the state gemstone for Texas, and Texas is one of the few regions in the United States where blue topaz can be found naturally. Pretty much all blue topaz on the market has been heat treated to get its brilliant hues, but if you are lucky, and you enjoy digging in the dirt, you might just find something truly exceptional.
So how does Topaz form? Why is Texas one of those special areas where topaz can be found? Well, in order to understand topaz, you need to know a little bit about how continents collide.
Think of the surface of the earth as a bunch of floating islands. There are two types of islands - continents (all the landmass you live on) and oceanic plates (the rocks at the bottom of the sea). Over time, these islands move and bump into each other. Geologists call these collision events orogenies.
A lot of things happen during an orogeny, including the building of mountain chains and volcanoes. The types of mountain chains will differ depending on the type of orogenic event. An example of two continents colliding is the Himalayas, which has formed the highest mountain chain in the world. When a continent collides with an oceanic plate, mountain chains and volcanoes like the Chilean Alps form.
Below the surface of these mountains, a whole lot of heat is generated and the rocks at the contact between the two islands begin to melt and form what are called magma pools. Sometimes these maga pools are extruded through volcanoes in explosive eruptions and flows that become basalts and other mafic rocks. Other times, the magma sits in the earth and slowly cools to form granite. The latter kind of rock body is where topaz is born.
As I already stated, Topaz is part of the silicate family, which means it has a lot of silica in it. Silica is one of the most common elements on earth, and the mineral most people are familiar with is quartz. So, it should come as no surprise that topaz is found along side minerals like quartz and feldspar. Granites are made up of a lot silica, and because they cool quite slowly compared to extrusive basalts, they are coarsely crystalline. Sometimes those crystals can get quite large! Ever see those counter tops that have crazy square shaped pink crystals that look like their just floating in the rock? Those are a special kind of granite called pegmatite.
Topaz are most typically found in pegmatites because these kinds of granites form late when the granite body (aka batholith or pluton, as we geos call them) cools. Pegmatites are also important sources of other rare earth elements, such as flourine, lithium, tin, thorium, and uranium. The reason these elements are concentrated in pegmatites is because as the granite cools, it is crystallizing all of the more common minerals into rock, leaving behind the rarer earth elements in the magma. This magma also has a lot of water, which under certain pressure and temperature conditions can create a chemical reaction where the remaining components in the liquid quickly crystallize, paradoxically generating very large crystals. Many pegmatites cross cut their original granite bodies as veins or dikes, but don't feel fooled as these veins can be quite large on their own.
The origin story for Texas topaz sings this tune - it formed as part of a granite body that developed after an orogenic event 1.2 billion years ago, called the Grenville Orogeny. Southern Texas has been a coastline for a very long time, that is when it was not part of a mountain chain. Over and over again, the south eastern margin of the United States has been marrying and divorcing Africa or some other continental mass through the cycles of plate tectonics. 1.2 Billion years ago, the earth was undergoing a mass collision, where all the continents were coming together to form a supercontinent, call Rodinia. Do not confuse Rodinia with a later supercontinent Pangea. Did you know there earth has formed more than one super continent? I'll tell you about that in another post.
Texas topaz can be found as part of a granite batholith called the Llano Uplift. On a geological map, it looks a bit out of place surrounded by bands of younger strata. It's called an uplift because after the granite was emplaced, the region around the Llano area subsided, creating a natural topographic high that persisted for many millions of years. Today, the region is quite hilly due to the erosion of the granite and surrounding bedrock. The most famous geologic formation associated with the Llano Uplift is Enchanted Rock, a large exposed granite dome just outside Fredericksburg, TX. GIA's excursion went to the small town of Mason, where it is possible to hunt for topaz on private property with permission. We visited Lindsay Ranch, a neat little homestead on 300 acres of rolling farmland.
Part of the thrill of rock hunting is that you don't know what you will find. There is a bit of an art to the process too. For example, topaz in the Llano area tends to weather out into surrounding streams and because it is very dense, will drop to the bottom of current edies. Therefore, when searching for topaz, you often need to dig a little. Alternatively, if you can locate the dike where the topaz is most concentrated, there might be fragments weathered out on top of the hills. I attempted hunting in both areas, and although I came up empty handed, I did find some other cool minerals like mica and garnet! We were rained out in the afternoon, but I was not deterred - I hope to return again to see what I can find.
If you are ever in Texas and thinking about trying your luck topaz hunting, I highly recommend reaching out to the local rock shops or searching online for excursions. If you would like to learn more about the history of excavating topaz in Mason, Texas here is an interesting article: https://texashillcountry.com/texas-treasure-hunt-topaz/
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