Adding more fire to diamonds with atomic age technology

It looks like any jewelry store: long, brightly lit glass cases sparkle with gold watches, bracelets, and rings.

At the rear of the salesroom here, though, sits irradiation expert Harry Nieman. He slides a small brown packet from his desk drawer. ''I've got to send this one off,'' he remarks, his opened hand revealing a brilliant, 7 3/4-carat diamond that stuns the eye with its blue-green color.

That color, rare for a diamond, is not the work of Mother Nature. It's the work of Mr. Nieman's company, Nu-Age Products, which is in the business of enhancing the color of gemstones through a man-made process. The stone, worth about $15,000 before treatment, would now be worth about $20,000.

Each year, at commercial and university facilities around the world, millions of dollars worth of gems are subjected to high levels of radiation. The result is rarities like canary-yellow diamonds and azure-blue topaz - at relatively reasonable prices.

But recently, the dangers of this atomic age technology were revealed: A batch of gemstones from Brazil was found to be radioactive.

George Rossman, associate professor of mineralogy at the California Institute of Technology, examined the contaminated lot and found that ''they were quite noticeably radioactive.'' Tests proved that some of the semiprecious stones exhibited radiation levels 60 percent above normal background levels. Although the danger of wearing such a stone (none has ever gotten to the retail level) is a topic of dispute, Dr. Rossman says a prudent person would avoid them.

Irradiation, as it's practiced in the US, is safe, says Rossman. The two processes used - gamma ray and electron beam - are tightly controlled and are not conducted at high enough energy levels to produce materials with significant radioactive qualities.

But attracted by glittering profits being made elsewhere in this business, suppliers in gem-rich Brazil tried to cash in by irradiating at home. Rossman theorizes that the stones were ''unknowingly and/or unscrupulously'' placed near or inside a nuclear reactor, which subjected them to high-energy neutron bombardment.

In the last year and a half, two other batches of stones - topaz from Brazil - were also found to be radioactive. These discoveries have startled the gemstone industry, prompting some gem wholesalers to acquire Geiger counters to help detect excessive amounts of radioactivity.

Irradiation is not new. Gems have been color-enhanced in this country since around 1940, says Nieman, who estimates 75 to 80 percent of all colored diamonds and blue topaz on the market have been treated - something that generally can be detected only by gemologists with the aid of spectroscopes.

Other gems that are being irradiated, according to Robert Crowingshield, vice-president of the Gemological Institute of America, include amethyst, citrine, quartz, and pearls. He says stones are irradiated in Europe, the Soviet Union, and Japan.

Martin Bressler is an engineer at IRT of San Diego, Calif., which operates one of the few electron-beam facilities used for irradiation in the US. He explains that the color of a gemstone ''is due to the fact that it was irradiated in the earth over millions of years.'' Radiation treatment speeds up that process to hours or often minutes.

But, he adds, ''when you irradiate, the higher the energy level, the more likely you are to create radioactive elements with significant half-lives.''

Mr. Bressler explains that the electron-beam process also uses high energy levels. But, he adds, ''we never let things leave the building unless they have have been . . . cooled down to a level below the natural background radiation.''