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A Simplified Guide to Buying Loose Gemstones
If you're looking to buy a loose gemstone at auction, you've come to the right place. Our experts are here to help.
F or even the most seasoned enthusiasts, buying jewelry at auction can feel intimidating. So for mere mortals, the prospect of purchasing a loose gemstone can feel like untying a Gordian knot. Understandably so—few novices can inspect a sparkling stone and instinctively discern its quality, rarity, or value. And, unlike signed jewelry or fine art where you can often rely on the signature to offer a hint, loose gems don’t offer the same signposting.
Walk into one of the many gemstone exhibitions around the world—the Tucson Show, Gem World in Hong Kong or Gem Genève—and you will leave with your eyes glazed over, looking for the nearest watering hole. Giant halls filled with hundreds of booths teeming with extraordinary stones, from the exceedingly rare to the fantastically expensive. If you’re not in the business or have a trusted advisor, how do you know if you’re paying a fair price?
Auction is the ultimate fair marketplace. There is no set price. A global audience of bidders determines what something is worth. At Sotheby’s, you also have a team of seasoned specialists who can speak to the strengths—or weaknesses—of a stone and help define your priorities. Color over clarity? Quality over size?
Auction is also an invitation to acquire gemstones from sources that are now depleted: “old mine” emeralds, Kashmir sapphires and Argyle diamonds. Or perhaps you want a stone with history: an emerald from the wreck of the Atocha, the Beau Sancy or the Farnese Blue?
Whether you're seeking a stone to create a one-of-a-kind piece or building your collection, we've put together a few tips to buying loose gemstones at auction.
The first and most important step in the process is a personal one. Does this gemstone capture not just your eye, but your heart? If the answer is yes, the rest will follow, but you need an experienced guide to ensure your heart doesn’t run away with your wallet. Acquiring a gemstone at a reputable auction house can be not only an enjoyable way to build your collection but a smart one.
For some, buying a loose gemstone is infinitely more rewarding than buying a ready-made jewel. “I compare this to a house you are looking to buy. Are you the type of person who wants to do renovations and really make it your own? Or would you rather have a move-in-ready property where all of the work is done and you are buying a completed piece?” says Haley Abbott, Jewelry Specialist at Sotheby’s New York. “This really comes down to personal taste. Some people find it fun to work with a jewelry designer and create a unique, bespoke piece. Others gravitate towards buying a signed stone piece from a prestigious house, such as Cartier, Van Cleef & Arpels, Tiffany & Co., or Bvlgari.”
The mystery—and the thrill—that surrounds loose stones is an addictive element. “I love when someone inherits a piece and knows nothing about it. It's as if we are putting the pieces of the puzzle together. The surprise on their faces when I tell them their stone is worth much more than they ever thought brings me sheer joy!” she says. “No two colored stones are the same. Natural precious colored stones are something that can’t just be recreated. If you miss out on a stone that's rare, who knows if or when you may see one that’s similar again. That’s the beauty of them, and what makes them so special.”
If you aren’t a gem expert (and even if you are) the first place to start is with your eyes. This might seem obvious, but many prospective buyers become so focused on gemological reports they forget they’re buying a jewel, not a piece of paper.
That said, gemological certificates have become increasingly important in today’s market. Most gemstones of significant size offered at Sotheby’s are accompanied by certificates “Certificates can be really overwhelming at first, as there’s a lot of information to take in,” says Abbott, “but it’s our job to explain the certification, and we are here to help.”
For diamonds, the Gemological Institute of America (GIA) is the most widely accepted lab. Their certificates provide information on color, clarity and other key factors that go into determining a stone’s value. Colored stones, with their many subtleties, are more difficult to quantify. The labs used by Sotheby’s include the American Gem Laboratory (AGL), the SSEF and Gübelin. Here, country of origin and the presence of treatment are most significant.
Value for diamonds and colored stones alike extend far past their size to include color, cut and treatment. “Emeralds, for example, are naturally more included compared to an aquamarine,” says Abbott. “Generally speaking, however, the clearer and cleaner the stone, the higher the price it will fetch.” It is helpful to note the quality of the cut, too. “I find the cut of a colored stone to be one of the most underrated factors in its appearance. The better the cut is, the more ‘alive’ it looks with the light bouncing around off the facets of the stones in the perfect way,” she explains.
White diamonds are the closest thing to a commodity in the gemstone world. Diamonds of specific sizes, colors and clarity tend to trade within a prescribed value band. This band is set by the Rapaport Report, a chart that tracks prices paid in the wholesale market and widely used by the industry. There is nothing that compares to this in the world of colored stones.
While depth of color and clarity are central considerations, a colored stone’s country of origin can have a tremendous impact on value. Much of this is tied to tradition. For centuries, the most highly prized emeralds have come from Colombia. For rubies, Burma (Myanmar) is the country of choice, and for sapphires, it’s Kashmir. Country of origin is so important, in fact, that an inferior stone from one country might command a higher price than a very fine example from another.
“The reason why country of origin, or provenance, is so important is because in certain cases, such as with Kashmir sapphires, there was only one mine producing these stones for just ten years in the 1880s, making them incredibly rare, and it's fascinating to see how strong they hold in the market due to this rarity,” explains Abbott. “That said, just because a stone comes out of a specific location doesn’t always mean that the stone will be extraordinary—it’s still crucial to see a gemstone in the flesh.”
“I’ve been excited to see Paraiba tourmalines having more of a moment recently. The neon blues and greens that are characteristic of these stones are completely captivating. We had a collection back in June that included a group of loose Paraíba tourmalines, and they had exceptional results. Brazilian origin Paraíba tourmalines like those are very rare due to the original mines they come from being largely depleted decades ago, and the material only being mined for about five years during that time.”
Because top-quality colored stones from traditionally prized regions are increasingly rare and sought-after in the market, stones from newer sources have gained momentum. Africa, known as the world’s largest producer of diamonds, has been producing exceptional specimens, including Paraíba-type tourmalines and rubies. The highest price ever paid at auction for ruby is not from Burma, but Mozambique.
Before investing in a loose stone, you should always check its certificate for whether it has undergone any treatments. Colored gemstones commonly undergo heat, resin, or oil treatments to improve their color or clarity, and the nuances matter. Heat treatment involves firing gems to high temperatures to alter trace elements and crystal structure to improve color uniformity or heal tiny fissures. Emeralds are sometimes soaked in oil or resin to fill inclusions. “Generally speaking, we find that oil-treated emeralds fetch a higher price than those treated with resin because this is the more traditional method and, unlike resin, oil can easily be removed,” Abbott explains.
That said, with certain types of gems, such as sapphires and rubies, the majority of stones on the market are heat-treated. “People have been heating sapphires for hundreds of years, so it’s a routine process, but it does mean that when we see a rare, unheated Burmese ruby, it’s going to skyrocket at auction,” she adds.
Diamonds can also be treated, often with radiation, to improve or change their color. If you’re looking at a green diamond, chances are it’s been treated in a lab. If the GIA certificate says the color is natural in origin, then you have something truly special. The clarity of a diamond can be improved, through the use of lasers. This will be noted in the comments section of a GIA certificate and easy to miss, so look closely. While it improves the appearance of the stone, the value is significantly less than a diamond of the same clarity that hasn’t been treated.
To evaluate whether a stone is fairly priced, the key is not to rush your research. “At Sotheby’s, we always align our prices fairly with the market, but there are things to keep in mind,” says Abbott. “You must remember to compare prices with the secondary market rather than a wholesaler or retailer, as these are very different,” she explains.
Ultimately, rarity is the number one factor in influencing a gemstone’s value. “Nature created these stones, they aren’t mass produced in a laboratory, which is why lab-grown gemstones have very little re-sale value,” she explains .A 3.00-carat diamond of D color and Flawless clarity is far rarer than one that is J color and SI2 clarity. It’s also about five times the price.
While most retailers offer gemstones cut to precise, modern standards, a premium is often paid for the charm that comes with a stone cut decades or even centuries ago. This is especially true at auction where the buyers are often seeking something they can’t find at a high street store. “We are now seeing older diamonds from important collections such as Old Mine cuts sell for higher prices per carat than modern stones with the same specs,” says Abbott.
When trying to evaluate whether a stone is worth the estimate, Abbott believes it’s possible to assess a diamond with a trusted GIA report against market rates without having to see it in-person, but to be wary with colored gems. “I never suggest buying them without handling them first, because the color can look very different in pictures.” Abbott bought a loose stone for her own engagement ring, so her experience is both professional and personal. “I looked at over 100 loose sapphires before I decided on mine. The color was much more saturated than some of the inkier darker blue sapphires I had always traditionally seen.”
The monthly Sotheby's Gem Drop is an excellent place to start when you're thinking about acquiring a loose gem. Often sold loose or simply set, these stones are offered to those with a sense of adventure and a desire for something truly custom. Our Gem Drops have even included pieces from Jogani and Kindred Lubeck (aka the woman who designed Taylor Swift's engagement ring).
If you don't have a designer in mind, we can help: Sotheby’s now offers Bespoke a setting service, where you can either design a piece of jewelry using a stone they have already bought at auction, or commission a design and ask the sales team to source a suitable gemstone to their exact specifications. The service partners with elite workshops across the US, Europe and Asia, many of whom craft jewelry for the world’s major maisons.
While many brands require a twelve-month lead time and have strict design guidelines in place for their bespoke services, Sotheby’s clients have full creative control and can expect their creation to be crafted in significantly less time, depending on the complexity of the design.
