In 1942, scientists at the University of Michigan put some diamonds
in a cyclotron and zapped them with heavy radiation to produce vivid green stones.
After a brief quarantine period during which residual radioactivity disappeared, the world had its first artificially colored diamonds that were safe to wear.
These diamonds were purely experimental. Commercial stones colored by irradiation weren't introduced in large numbers until the 1950s. And when they were, the market for naturally colored diamonds crashed because there was no simple test to distinguish hues created in nature from those created in a lab. In short, the danger from irradiated diamonds
was to wealth not health.
Fortunately, gemologists discovered a simple test using spectroscopes to tell natural from irradiated diamonds (each shows different spectra, or light absorption characteristics). By 1960, the prices of natural colored diamonds had fully recovered and were at the beginning of an upward price spiral that lasts to this day.
Although very high-energy forms of radiation have been used to color diamonds in the past, today the process is very benign—usually involving low-dose electron bombardment with a linear accelerator that penetrates and colors the thick outer area of the stone. Safe and permanent (unless subject to very high heat from, say, a jeweler's torch during repairs of a setting), irradiated diamonds have become a staple of the modern diamond jewelry market.
There are two reasons for the popularity of irradiated diamonds
First, irradiation (plus heating afterwards) produces affordable colors.
Natural deep-yellow diamonds are very expensive. Natural greens, blues and reds are prohibitive in cost.
But even if could afford the tens and hundreds of thousands of dollars per carat to pay for these colors, you will be hard-pressed to find them. Which brings us to the second reason for irradiation: rarity.
Thanks to irradiation, consumers have their first chance to see, let alone buy, diamonds with lovely kiwi and Bordeaux colors in their local jewelry stores. In addition, irradiation has been used to produce truly black diamonds that rival black onyx for beauty. Natural black stones are often blemished and pitted.
Occasionally, overdosed diamonds have been imported into America. Most didn't get very far because Customs or advanced gem labs caught them. But you don't want to be one of the handful to have been offered one of the handful of tainted gems that got through.
All in all, irradiation has helped bring the diamond rainbow to a broader buying public. These stones are safe, color-fast and affordable. But buy them from reputable sources who disclose the process by which these gems attain their final hue.