The Rocks Know

A topaz crystal Image Credit: Rob Lavinsky, irocks.com

Featured Image: http://commons.wikimedia.org/wiki/File:Quartz-Topaz-tch03a.jpg Caption: A topaz crystal Image Credit: Rob Lavinsky, irocks.com

This week’s Mineral Monday delay wasn’t due to a turkey-and-other-excellent-food-coma, whatever you may think. It’s because this week’s mineral is topaz, November’s birthstone, and I wanted to wait until my mom’s birthday to post it.

Topaz  (Al₂SiO₄(F,OH)₂) is another silicate mineral along with kyanite, olivine, and oh, so many more. In its pure form, it is colorless and clear, and can be cut to rival diamonds.

Image Caption: The Chalmers Topaz at the Field Museum, a near-colorless 5889.5 carat topaz (Image Credit: Opacity via Flickr)

Usually, when someone refers to ‘topaz’ people think of the warm brownish-yellow common to many varieties of topaz around the world. This classic topaz color is often confused with citrine, a type of quartz.  Etymology nerds the world over are still arguing over the origin of the word topaz, with some maintaining that it came from the ancient name of an island on the east coast of Egypt, called Topazios, where the gem peridot was mined. Others maintain that the name came from the Sanskrit word tapas meaning “heat or fire.”

Like many gems, topaz comes in a wide range of colors, from blue to green to pink. But topaz is cool because it can change colors. Historically, people noticed that the vibrant colors of topaz specimens would fade over time, especially if frequently worn in sunlight.

More recently, gemstone experts realized that heating and irradiating topaz stones could cause dramatic color shifts in topaz. A blast of radiation can be fairly destructive on an atomic level. That burst can knock loose atoms within the crystal structure, and also release a few electrons in the process. If an electron fills the hole left by an atom, it can absorb certain wavelengths of light, resulting in a blue or yellow color.*

Blue Topaz Image Credit: Chris Kohtz via Flickr

Heating reverses this process, releasing the electrons, and resulting in a colorless stone.** Gemologists can use varying types of radiation and heating processes to manipulate a topaz’s color dramatically

This process has become so common that it has even affected prices for gems. There’s really no way to tell whether a stone is naturally blue or irradiated, and so blue topazes, once rare, have now become relatively inexpensive.

Happy birthday Mom!

*Jill Banfield at Berkeley has a great description of the process on her Gems and Gem Materials class website: here.

**Colorless, or a stone colored only with impurities instead of loose electrons. See more info here and here.