Consumer Reports agrees; a recent article stated that buyers of diamonds and other precious stones are often paying 50 percent more than they should.

The felicitously named Stone is trying to do something about that. After 14 years of managing the precious jewels department at Neiman Marcus in Northbrook and working in the fashion jewelry department at Saks Fifth Avenue on Michigan Avenue, she started her own business two years ago. At Engaging Rings she counsels individuals on how to buy top-quality diamonds, rubies and precious metals as well as how to find the best deals. (Contact her at www.engagingrings.com or e-mail Dana@engagingrings.com.)

Stone has a BFA in jewelry design and metalsmithing from the University of Kansas and holds a graduate gemologist degree from the Gemological Institute of America.

Purchasing a diamond engagement ring, or any diamond, can be an exercise in frustration for the inexperienced buyer, Stone says, adding that there is more to look for than the traditional "four C's" (cut, carat, clarity, color). Here's some of her advice:

Ask the seller if the stone is florescent. "If it is more than slightly florescent, the stone can look waxy or dull," she says.

Always compare at least three stones side by side to get a feel for the material. "Diamonds can have the same color and clarity but look very different due to the cut or material caliber," she says. "If you have nothing to compare a stone to, the untrained eye finds it difficult to see differences."

Use a 10x loupe when examining your potential diamond. "Even if you don't know how to use one, the jeweler should teach you," she says. "Keep the loupe one inch from your eye and the diamond one inch from the loupe. If you can spot some inclusions (foreign substances within the diamond) in the loupe and know were they are, then check with your naked eye to see if you can detect them without magnification."

Ask if the diamond comes with an independent lab certification. "Some labs are more credible tan others. GIA (Gemological Institute of America) is the most reliable one in the industry," Stone says. "GIA-certified diamonds also command the highest price. It's possible to buy a valuable diamond without a GIA certification; you just need to trust your jeweler." Diamonds don't always fall into neat, precise categories, she adds; "every grade is a range and an opinion." Most important: "Makes sure you're buying from an educated, experienced jeweler."

That last piece of advice is one reason that Stone counsels against buying a diamond over the Internet, which many consumers are doing. She likens it to "playing Russian roulette."

"I find it so disturbing that diamonds, the most beautiful and valuable materials on earth, have been degraded to a commodity, like pork bellies," she says.

Stone notes that even with her years of experience inspecting and grading diamonds, "I would not blindly buy a diamond over the Internet. Yet inexperienced Americans are spending up to $50,000 for a diamond sight unseen," she says.

"You can have two diamonds with the exact same carat weight, color and clarity, yet one can be far superior to the other," she says. "How the stone is cut and its proportions have a large effect on its beauty. GIA has come out with a cut grade, but that is only for round brilliant stones. How does the average person tell if a fancy shape is well cut?" The short answer, Stone says, is they usually can't.

In addition, she says, most gem-quality diamonds contain a number of impurities that are not listed on the grading certificate and are not easily detected by the untrained eye. The average buyer, she says, "would not be able to detect an off make, see that a stone is florescent or that it doesn't have the scintillation that it should, especially when that inexperienced eye has nothing to compare it to."

Yet, she says, people are buying diamonds over the Internet all the time and not knowing that they might have received an inferior stone to match the inexpensive price, rather than a good deal. "There are so many scams over the Internet that there's not enough time to list them all," she says. "But the thought of receiving a cubic zirconia (synthetic stone) or fracture-filled stone should be enough to scare anyone off."

In addition, Stone says, customer service-for such necessities are sizing rings and fixing loose stones-is often non-existent with Internet companies, or the customer must send the ring back and be without it for weeks.

Stone says that while she shops for such items as books and DVDs over the Internet and realizes that online marketing has its place, "it's best for people to leave the more perplexing merchandise to an expert they trust. Think about your occupation and what your years of experience offer. Now think about someone with no experience being upgraded to an instant expert over the Internet," she warns.

In other words, the diamond you receive sight unseen may definitely not be a girl's-or a guy's-best friend.