Zapping zircons

Fans of Physics Today‘s Facebook page occasionally send me messages, most of which are requests for more information about something to do with physics. The one I received on Monday was no exception. A fan from Jordan wanted to know about research in “gemstone treatment.”

Not knowing what he meant, I Googled the phrase, which led me to a website touting the value of untreated gemstones. Some gemstones, I found out, are routinely subjected to heat, chemicals, and even ionizing radiation to change or improve their appearance.

To achieve its so-called super blue color, this topaz has been bombarded with high-energy electrons from a linear accelerator.

Not having heard about the irradiation of gemstones, I investigated further. One of the first documents I came across, thanks to Wikipedia, was Charles Ashbaugh’s “Gemstone irradiation and radioactivity ,” which appeared in the winter 1988 issue of Gems & Gemology.

When he wrote the article, Ashbaugh was an engineer at UCLA’s nuclear energy laboratory. His article is worth reading—not only for its review of how both natural and artificial radiation sources alter the optical properties of gemstone minerals, but also for its tutorial on radiation (the sidebar on the various radiation units, with its analogy to sun bathing, is exemplary!).

If you’re like me, you probably knew that amethysts, emeralds, and other gemstones owe their colors to the dilute presence of impurities. Ruby, for example, consists of an aluminum oxide (Al₂O₃ crystal) doped with chromium atoms. From Ashbaugh I learned that irradiating a gemstone with gamma rays, high-energy electrons, or neutrons transmutes the impurities, thereby changing the wavelengths absorbed by the crystal. Naturally pale blue topaz can be turned a deep “super blue.” Colorless zircon can be turned pink.

As you might expect, irradiation could make a gemstone radioactive. In 1988, when Ashbaugh wrote his article, the regulatory status of irradiated gemstones in the US was confusing, inconsistent, and subject to state and federal jurisdiction. It was easier for a US jeweler to legally obtain irradiated gemstones from abroad than from the US. The regulations are clearer now. In fact, now that there are more irradiated gemstones on the market, the Nuclear Regulatory Commission felt the need last year to issue a fact sheet , whose summary succinctly states (in bold font):

• The NRC believes irradiated gemstones currently on the market are safe.
• The NRC has not requested that jewelers take these stones off the market.

Does irradiation diminish the allure or value of gemstones? Not for me. For one thing, a perfect diamond crystal consists of identically arranged carbon atoms. If you could make one in the lab, it would be identical and indistinguishable from a perfect natural crystal. Structural perfection, not naturalness of origin, is a crystal’s paramount property.

What’s more, it doesn’t matter to me whether a tourmaline acquired its color through millions of years’ exposure to natural radiation emanating from the surrounding rock or through a few hours’ exposure to 1.17- and 1.33-MeV gamma rays from a cobalt-60 source.

Ashbaugh’s article is illustrated with several photographs of beautiful, gleaming gemstones in a variety of colors—which prompts another question: If you can make, say, a deep red gemstone by irradiating any one of several naturally transparent, colorless crystals, does it matter which crystal you start with?

The answer could be yes—if you care about how much a stone sparkles. Whereas a natural emerald’s refractive index is 1.6, an irradiated green diamond’s is 2.4. Until a crystal’s refractive index can be engineered, I suspect diamonds will remain the most prized gemstones.

As for the Jordanian Facebook fan who wanted to learn about gemstones, it turned out he was really interested in crystal healing . I couldn’t help him.