Radiation, Heat -- Improve Nature's Gems

Caltech News Release via California Geology (1/79)

Lithosphere (June 1993); Fallbrook Gem and Mineral Society, Inc.; Fallbrook, CA

As the demand for gems rises and the supply diminishes, technology is rapidly developing to duplicate and even improve on nature's handiwork. The practice of improving gemstone color by subjecting the minerals to radiation, or altering their natural radiation by heat, has become increasingly common during the past two years and will continue as naturally attractive stones become scarcer, according to Dr. George R. Rossman, professor of mineralogy at the California Institute of Technology.

Impurities In Gemstones

Gems derive their beautiful hues from mineralogical impurities. For example, jadeite, NaAl[Si2O6], (one of two minerals commonly called jade) is green because it contains chromium, an element that absorbs red, blue, and violet, but not green, light waves. Iron also absorbs these hues, but its absorption capacity is weak compared with chromium. For this reason nephrite, Ca2(Mg,Fe+2)5[Si8O22](OH,F)2, (a mineral called jade but with iron as its impurity) is a pallid green compared with the deep green of the jade that contains chromium.

Beryl, which is colorless in its pure mineral form, Be3Al2[Si6O18], becomes emerald with chromium impurities. With manganese impurities beryl becomes pink morganite; with iron, aquamarine. In aquamarine, the impurities that cause the colors occur in atom-sized, straw-like channels in the mineral's structure.

Gamma Radiation

The most common means of altering gemstone color by radiation is with gamma rays (cobalt 60 radiation). Gamma rays release electrons from their normal location in the gem. The color change depends on where the electrons relocate and on the charge of the atoms near them. These factors control the way the stone absorbs light, and thus they dictate its color.

There are no guarantees of success for gemstone enhancers because it is impossible to predict how irradiation will affect a stone. Certain mineral impurities must be present and the radiation must shift electrons to desirable color-producing locations. Normally, many gemstones must be irradiated in order to obtain a few stones that are altered to the desired color.

Topaz, Al2[SiO4](OH,F)2, which is colorless in its pure mineral form, is one gemstone suitable for irradiation. When exposed to cobalt 60 radiation, the stone may change to a cinnamon brown color, but this tone normally fades over several months when exposed to light. Recently, however, gem dealers have developed the technology to produce a brilliant and permanently blue topaz through irradiation, equal in its attractiveness to rare natural blue topazes. An irradiated blue topaz exhibits all the chemical and optical properties of the natural stone, and can be distinguished only through an analytical process of measuring the light emitted by a gemstone when it is heated.

Irradiation is also used to change quartz (clear and colorless in its pure mineral form, SiO2) to the more attractive and valuable smokey quartz. But this transformation only occurs when the colorless quartz contains traces of aluminum, and ultimate success depends on where the electrons settle after irradiation. Amethysts can be produced by irradiating quartz that contains iron impurities, but attractive amethysts occur so abundantly in nature that it is not commercially economic to create them synthetically.

Diamonds, C, which occur naturally in many colors, can be irradiated to produce different hues. Irradiation can change lightly tinted diamonds to brilliant yellows or greens. However, the dealer irradiating such expensive stones as diamonds is taking a heavy risk, since a valuable, lightly tinted diamond may turn out a muddy brown.

Not all artificially induced color changes are permanent. Kunzite, a lavender variety of spodumene, LiAl[Si2O6], emerges a brilliant green if bathed for 15 minutes in cobalt 60 radiation. The radiation changes the electrical charge of manganese ions responsible for the gem's color from +3 to +4. But when exposed to light, the +4 ions begin to add more electrons to their structure and the stone reverts to spodumene, drab and colorless in appearance.

Heat Treatment

Irradiating gemstones to change their color is a relatively new process, but altering their internal structure by heat has been practiced for many years. Stones are often heat-treated at the mines by persons who developed simple procedures through experimentation.

Among the stones that benefit from heat treatment is aquamarine, which occurs naturally in many colors -- yellow, yellow green, blue green, and blue -- depending on the atomic charge and location of the iron atoms in the mineral. Heat that changes the atomic charge of iron atoms from +3 to +2 transforms a yellow aquamarine into a more valuable blue aquamarine.

The preceding article was published in the June 1993 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).

The material is in the public domain, and may be republished freely.

Last updated: 18 September 2002