Colorado’s True Blue Gem

By Jack Thompson, Kay Thompson and James Hagadorn

Blue topazes from Glen Cove are on display in Coors Gem and Minerals Hall.

Pikes Peak looms large across Colorado's landscape. This world-renowned mineral collecting site formed over a billion years ago as molten rock percolated upward through our mountainous crust. Its granite also makes fine-looking countertops!

People have been picking up crystals from crumbling exposures of Pikes Peak granite for millennia, often rooting around in the geode-like cavities that dot the mountain. Such fissures are commonly lined with smoky quartz, amazonite, fluorite and topaz. Among these, topaz is the only mineral that is commonly cut, or faceted, into gemstones for jewelry.

Topaz is a clear mineral containing aluminum and silicon (Al2SiO4(F,OH)2) that forms during the last moments when underground lava, or magma, cools. As the magma solidifies into granite, hot fluorine-bearing gases work their way through crevices in the rock, catalyzing the growth of topaz along the walls of cavities and fissures.

Topaz has no industrial uses but is commonly used in jewelry because it comes in many colors and is relatively easy to cut. It is harder than quartz but not quite as hard as sapphire. For those of you born in November, topaz is one of your birthstones. Some exceptional topazes have chromium or iron impurities in them, which give them pink-red-purple colors. Topaz that has been exposed to radiation becomes yellow, brown, or blue. Most of the blue topaz sold by jewelers is "common" clear topaz that was irradiated in the laboratory. "Natural" blue topaz is very rare.

The cliffs above Glen Cove, along the northwestern flank of Pikes Peak, contain some of the best natural blue topaz in the world. Specimens more than a pound in size have been collected, and many tend to have complex crystal shapes. These crystals were first discovered by Luther McKnight in 1944 as he was photographing wildflowers near the rubble at the foot of the cliffs. Yet owing to the steep terrain, only a half-dozen mineral collectors have successfully prospected its treasures since.

One of them was John H. Alexander, son of the Alexander film and airplane manufacturing family of Colorado Springs. A geology major at Colorado College, mineral collector and mountaineer, he hung by ropes to mine topaz from the mineral-studded cavities of the Glen Cove granite cliffs. One of his prize finds was a clear sky-blue specimen that was faceted into a brilliant 103.2 carat round gem. Alexander's widow donated it and other blue topaz specimens to the Museum in 1978, adding some of Colorado's rarest gemstones to our research collections.

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