It is not uncommon for gemstones to be exposed to radiation in order to change their color and to increase their market value. Color changes by irradiating stones is legal, but they must be stored for a set time prior to release to let radioactivity decrease.
Gamma RadiationThe 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, AI2[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 technology to produce 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 analytically 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 smoky 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 successfully.
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 irradiation 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, LiA[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 TreatmentIrradiating 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 have developed single procedures through experimentation. Since natural heating also occurs (ie: in volcanic areas), the artificial effects are sometimes indistinguishable from natural effects. In most cases, the results of heat treatment are permanent.
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.