The Gem Expert: Bill Craig Explains the Art of Gemology

What's the difference between an emerald and a garnet? How red does a corundum have to be to be called a ruby? How flawed is this diamond?

There are answers to these questions, but Bill Craig of Craig's Jewelry will tell you that in gemology, the study of gemstones, nothing is crystal clear.

"The art of gemology doesn't happen overnight – it's not an exact science," Craig said. "I've learned something new every day for 36 years."

The examination of gemstones runs in Craig's blood – his grandmother opened the shop 61 years ago, passed it along to his parents and now he is the president of the company.

"I enjoyed it when I was young," he said. "I got tired of asking my father in high school, 'What kind of stone is this?'"

So, at 18 years old, Craig went straight to gemology school to learn the ins and outs of the business and then went to work running the shop with his sisters.

Recently, he took his recertification test, administered by the American Gem Society, an ethical group whose goal is not to deceive the customer (nor to support any gemologist that might be inclined to do so).

Can you tell the difference between a used Rolex and one that's brand new? What do you do with a damaged diamond? These are the kinds of questions that come up.

Mostly, Craig said, it takes experience and a good eye.

"I really like the challenge of a colored stone," Craig said. "Ninety-nine percent of the time I get a gut feeling of what a stone is, but I still need to perform gemological tests to determine exactly what the stone is, in my opinion."

Much of what happens in gemology is about how light is affected inside the stone, which makes a difference in its appearance and luster.

And unlike the stones he examines, the world of geology is ever changing. Technology has made the detection of light performance more and more precise.

Craig took out an angular spectrum evaluation tool – it looks like a small, round microscope – and explained how you can see the light performance inside the stone.

To a layman, the reds, greens and blues in the image invoke a toy kaleidoscope, but Craig identifies the ideal properties of light.

"Basically, you want a lot of red, a little green, and symmetrical blues," Craig said, making it sound simpler than it is. "The question you want to answer is, 'What's going on inside the stone?'"

With two purple gemstones in front of him, a tanzanite and a dark blushing sapphire, he always knows exactly which is which when, to anyone else, there is no discernible difference.

"It's fun to pick out the small differences," Craig said. "It's always something new."