Fired-up color

A cornucopia of color, and an introduction to heat-treated gems, our photomontage includes (clockwise, from left) rings set with amethyst and golden citrine, accented by red garnets and near-colorless diamonds; “mint” light green tourmaline, set with spessartite garnets; blue zircon, embellished with contrasting yellow sapphires and rubellite tourmalines; and aquamarine.

Our suite of routinely heat-enhanced loose gems includes (clockwise, from left) an 18.14-ct. pear-shape golden citrine, a 10.85-ct. emerald-cut aquamarine, an 11.14-ct. oval brilliant pink topaz, a 13.20-ct. cushion amethyst, a 1.86-ct. pear-shape green tourmaline, a 5-ct. oval brilliant tanzanite, and a 6.05-ct. blue zircon.

In today’s world of retail jewelry, we take it for granted that a shopper should be able to go to any jewelry store and purchase a beautiful, crystal-clear, color-saturated gemstone of his or her choice. But leaving it to Mother Nature to provide these gems would be like going back to a time when supplies were so limited that only royalty and the aristocracy could afford them.

This month’s “Jewel of the Month” focuses on gems that have been enhanced by heat to improve (and sometimes even create) their customary colors, resulting in beautiful gemstone jewelry that nearly every jewelry lover can afford.

History. The phrase “history of heat treatment” conjures up a vision of a loincloth-clad man sitting cross-legged in front of a campfire, using a blowpipe to increase the intensity of the flame and cooking a gem or two. After all, if someone hadn’t stumbled upon the magic effect of heat on gems, most of us—and our customers—would never wear a gemstone. The knowledge of birthstones would be relegated to ancient astrological texts, and a gem like tanzanite would be so rare that its value would surpass that of ruby and diamond.

Heat treatment, however, has lost much of its former romance. Not many of today’s heat treaters sit scantily clad in front of a wood fire trying to alter the color of one or two gems. Most have modern furnaces, wear protective clothing, and perform enhancements on thousands of carats of gems at a time. What once was romance for the stone now is technology for increasing commerce.

Routinely heat-treated gems. Take a quick look around any jewelry store and you’ll see amethyst, citrine, ametrine, aquamarine, light green tourmaline, tanzanite, and blue zircon—all typically color-enhanced by heat. It is common to heat amethyst to create citrine, sometimes ametrine, and to heat-treat brown zoisite to create tanzanite. It is important to note that these color enhancements are, at present, probably not detectable, and some may never be detected. Other birthstone gems that fall into this category include demantoid garnet, pink topaz, Paraìba tourmaline, ruby and sapphire (which will be a separate topic in a later feature), and red zircon.

In the ornamental gemstone area, amber is commonly clarified by heat, and red tiger’s eye gets its red color from heat. In the chalcedony category, there would be no carnelian without brown agate and heat.

Black diamond is created by Mother Nature but can be imitated through technology. Using irradiation to create black diamond has been a common practice for decades. But more recently—and more inexpensively—heat treatment of bort (industrial diamond) has been used to create a commercial substitute for the natural color.

Value and price. The big question for the jewelry industry is how to deal with gems that have had their color enhanced—how to value them and how to distinguish between these and natural-color gems. If it were that simple, the industry could have figured it out by now. But there are different types of enhancements, and each can have different consequences for a gem and its value. If the enhancement can be distinguished, then the jeweler can make pricing distinctions. If, as in most cases, the enhancement cannot be discerned, then supply becomes the common denominator for determining the gem’s value.

Price lists for aquamarine, for example, assume the gem has been heated. The Guide states, “Most aquamarine is heated to remove the green component.” The desirability of greenish-blue vs. blue aqua is thus a matter of personal taste. There is no pricing distinction.

Apatite, commonly seen as a blue or green transparent gem, is heated to produce electric colors like those found in Paraìba tourmalines. Sometimes these gems are even referred to as “Paraìba-like apatites.” Price is increased as the color reaches full saturation. There is no evidence of heat, and no reduction of price due to heat treatment.

(Note: Because there is usually no way to tell whether or not one of these gems has been heated, prices for natural color and enhanced color are exactly the same. With corundum, however, there is a growing divide between prices for proven natural color and prices for heat-enhanced color. Because this is such a complex topic, it will be covered separately in another “Jewel of the Month” feature article.)

Care and cleaning. Color enhanced by heat in most varieties and species of gem is often stable under normal wear and repair. Some gemstones—tanzanite, for example—are more likely to be damaged than to suffer a loss or change in color (because of heat) during benchwork. However, there’s no guarantee that the color won’t be altered during high-temperature jewelry repairs, so it’s advisable to remove or protect the gem in question.

Recommended reading.Gems & Gemology, Winter 2002, “Chart of Commercially Available Gem Treatments,” by Christopher Smith and Shane McClure; and Gemstone Enhancement, by Kurt Nassau, Butterworths, 1984.

Log Out

Are you sure you want to log out?

CancelLog out