Explainer: Treated vs. Untreated Gemstones
Appraiser Katherine Van Dell mentioned “treated” emeralds in her appraisal of a beautiful circa 1900 emerald and diamond ring. What enhancements are performed on stones? And how do untreated gemstones compare to treated gemstones? Van Dell explains.
At the Salt Lake City ROADSHOW In August 2016, jewelry expert Katherine Van Dell appraised this ca. 1900 emerald & diamond ring for $6,000 to $8,000.
Apr 3, 2017
When ANTIQUES ROADSHOW visited Salt Lake City in August 2016, a lovely family heirloom caught the eye of jewelry appraiser Katherine Van Dell. The guest, Maurine, had been told by a jeweler that the ring was a costume jewelry piece. Van Dell had good news for Maurine: the ring was not costume, but made of real diamonds and a beautiful cabochon emerald weighing almost 20 carats. Furthermore, Van Dell was impressed with the emerald’s strong green color. “What you want in a gemstone is that vibrant color. You want to see green from across the room when you wear it.”
“What you want in a gemstone is that vibrant color. You want to see green from across the room when you wear it.”
Van Dell expanded on the color of the emerald, speculating that the precious stone was from Colombia, a very desirable source for emeralds, and that it may have been treated, as “they’re normally oiled to enhance color.” Appraisers have mentioned enhancing or treating gemstones on ANTIQUES ROADSHOW before, so we decided to follow up with Van Dell and find out more about treated versus untreated stones.
Here's What We Learned
ANTIQUES ROADSHOW: There are many methods used to treat or enhance gemstones: heating, bleaching, surface coating (such as oiling), dying — just to name a few. When did treating gems become commonplace?
Katherine Van Dell: Depending on the mineral, gemstones have been altered to improve color and clarity for thousands of years. For example, emeralds have been traditionally oil-treated or soaked with a kind of cedar oil solution for thousands of years to improve clarity. Depending on the mineral, different treatments have been available for hundreds of years or have become more available very recently.
AR: Are the gemstones of antique jewelry pieces therefore just as likely as contemporary gemstone pieces to be enhanced in one those ways?
KV: Again, gemstones have been synthesized, oiled and dyed for centuries, but in the case of a genuinely antique stone, it is less likely that it was treated to the same potential extent that a modern gemstone may be.
AR: Generally speaking, are untreated stones more valuable than treated stones?
KV: In the case of two stones of equal saturation of color and clarity, one being treated and the other not, the untreated stone is always going to be more valuable. Knowing that the vast majority of gems have been altered in some way by man, when a stone in its natural state is not needing of enhancement — that makes it exceptionally rare and likewise more valuable.
AR: Some gems are not usually treated, like spinel, garnet and tourmaline. Why?
KV: While it is true that these minerals are less often treated — usually because standard treatment methods like heating, for an example, are not as effective on gems like garnets — it is important to know that there are still man-made or "synthetic" varieties of these stones.
AR: Where can precious stones be assessed to determine if or how they have been enhanced?
KV: The Gemological Institute of America offers certification for diamonds as well as colored stones. The American Gemological Laboratory and the Swiss Gemological Institute are labs that also produce reliable certifications, detailing origin and enhancement levels of the colored stones.
(This interview was edited for length.)
