The main thing is to know what you don’t know. So I met Fiona Druckenmiller at her Upper East Side shop to  seek her best advice for buying gemstones. She’s got credibility: As one-half of the multibillion-dollar Druckenmiller fortune, she presides over FD Gallery, a six-story townhouse full treasures from the likes of Société Cartier, Van Cleef & Arpels Inc., JAR, Verdura, Hemmerle, and Viren Bhagat. Walk through the heavy, double-door portal and you’ll find yourself amid glittering items worth hundreds of thousands of dollars apiece. Her clients range from Upper East Side doyennes and French financiers to young, Asian entrepreneurs and Italian barons.

Druckenmiller wants to make it clear: “Just because you’ve bought an engagement ring doesn’t mean you know how to buy gemstones.” It’s solid advice, and a great place to start. Here’s what else you need to know if you are in the market for some seriously fancy jewelry.

You can’t eyeball quality

Do you know the difference between heated and unheated rubies? Can you spot an emerald that has been oiled, vs. one that contains no oil? Probably not. But those enhancements greatly affect the value of stones.

“People have been heating stones for hundreds of years to enhance the color,” Druckenmiller says. “But a Burma ruby that is unheated is going to be worth 10 or 20 times as much as a heated ruby from Africa.”

As for emeralds, many have little fissures in them, because emerald can be brittle. Jewelers often use oil to make the cracks less visible. So an emerald with no oil will naturally be more valuable than one that has been altered.

“It’s not something you can look at and tell,” says Druckenmiller. “People just don’t know. It’s not that they’re wrong to buy them. It’s just inexperience.”

The best way to find out what kind of quality you’re dealing with is to inquire, simply and directly. Question the seller as to where the jewel originated  and how it has been treated. Ask to see the certification papers. And bring along an expert, or a well-trained friend, to act as backup and offer a second opinion.

Even if you have a lot to spend, don’t assume Madison and Fifth avenues are the best places to shop 

While such brand-name emporiums as Harry Winston Inc., Tiffany and Co., and K. Mikimoto & Co. offer plenty of beautiful and expensive pieces, they also build additional costs into their prices as well. Glamorous stores require hyper-expensive real estate, with advertising campaigns and red carpet events costing millions each year. It’s all built into the cost structure of each big brand.

What’s more, these brands don’t necessarily offer the highest-quality stones. (Also keep in mind that lower-end big-box jewelry stores sell stones grown in laboratories, a practice so pervasive that this year the Diamond Producers Association launched a marketing campaign called “Real is rare” to emphasize the importance of genuine stones and to bolster business, especially among millennials.

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For instance, Mikimoto sells plenty of cultured pearls. The value of a cultured pearl can be just 10 percent that of a natural pearl. If you want the most natural, rarest item at the best price, you may want to look elsewhere.

“Learn as much as you can and find a trusted source, who also could help you learn,” Druckenmiller says. To find the best authentic items at fair prices, visit small, boutique shops and trustworthy private dealers, as well as auctions and estate sales.

At auctions, be persistent

If you do take your chances at public auctions and sales, be prepared to go up against dealers, retailers, and brand-name houses such as Cartier and Van Cleef, which often buy back important estate- and museum-quality jewelry to keep in their archives and exhibit worldwide. Why wouldn’t they? The supply of significant jewels and gem stones dwindles every year, thanks to inevitable loss, irreparable damage, and collectors who squirrel items into private coffers.

The major gem auctions—think Sotheby’s, Phillips, Bonhams, Christie’s—typically start in November in Geneva and run through December. Industry insiders who attend them “will be very involved” in the first group of auctions, Druckenmiller says. Then they retreat.

“When it gets to be a week before Christmas, they have likely already spent their money or they have gone on to other things—so you can find bargains,” she says. “This will even happen just at the later end of night sales, when they are dropping out, or their attention wanders. But you have to be lucky. It’s a matter of being really vigilant.”

Refuse to accept old or inferior certificates of authenticity

When you talk to a dealer or retailer, always request certificates of authenticity for any gem.