Education, Zircon

ZIRCON

MINERAL: Zircon
CHEMISTRY: ZrSiO4
COLOR: Blue, red, yellow, orange, brown, green
REFRACTIVE INDEX:
HIGH: 1.925 to 1.984 (+/- 0.040)
MEDIUM: 1.875 to 1.905 (+/- 0.030)
LOW: 1.810 to 1.815 (+/-0.030)
BIREFRINGENCE: 0.000 to 0.059 (low to high)
SPECIFIC GRAVITY: 3.90 to 4.73
MOHS HARDNESS: 6 to 7.5 (low to high)

Sources

China, Australia, Cambodia, Myanmar, Tanzania, Madagascar.

Birthstones & Anniversaries

Zircon is a birthstone for the month of December, along with turquoise and tanzanite.

Description

Zircon is a colorful gem with high refraction and fire that’s unfairly confused with cubic zirconia.
Colorless zircon is known for its brilliance and flashes of multicolored light, called fire. These zircon properties are close enough to the properties of diamond to account for centuries of confusion between the two gems.
Zircon occurs in an array of colors. Its varied palette of yellow, green, red, reddish brown, and blue hues makes it a favorite among collectors as well as informed consumers.
Colorless zircon is well known for its brilliance and flashes of multicolored light, called fire. These two zircon properties are close enough to the properties of diamond to account for centuries of confusion between the two gems.
Zircon occurs in an array of colors. Its wide and varied palette of yellow, green, red, reddish brown, and blue hues makes it a favorite among collectors as well as informed consumers.
Zircon crystals grow in many different types of rock and possess a range of optical and physical properties. Some zircons—usually green ones—display much lower values for these properties than others. Scientists have determined that the crystal structures of these gems were almost completely broken down by radioactive elements—often present in zircon as impurities—that damaged the gems’ crystal structure over long periods of geological time.
Some gemologists classify zircons into three types—high, intermediate, and low. A zircon’s classification depends on its properties, which are directly related to the amount of radiation-induced damage done to its crystal structure.
High or normal zircons have full crystal structures, with little or no damage from radioactive elements. As a result, they have the normal physical and optical properties associated with the mineral.
In intermediate or medium zircons, radioactive elements have caused some structural damage. They have physical and optical properties that are between high and low types.
Extensive crystal-structure damage from radioactive elements results in low zircons with much lower optical and physical properties. In extreme cases, they are practically amorphous, which means they lack an orderly crystal structure.
Virtually all the zircons used in jewelry are of the high type. Interestingly, radiation-induced crystal-structure breakdown can be reversed somewhat by heating zircon to high temperatures. High-temperature heat treatment repairs the stone’s damaged crystal structure.

ACCESSORIES

History

Many people have heard of zircon but never seen it. This is mostly because of colorless zircon’s wide use as a diamond simulant in the early 1900s. It was long ago replaced in that role by more convincing look-alikes, but its name still means “imitation” to many people. That’s unfortunate because zircon is a beautiful colored stone with its own fair share of folklore and charm.
In the Middle Ages, this gem was thought to induce sound sleep, drive away evil spirits, and promote riches, honor, and wisdom.
Many scholars think the stone’s name comes from the Arabic word zarkun, meaning “cinnabar” or “vermilion.” Others believe the source is the Persian word zargun, or “gold colored.” Considering zircon’s color range, either derivation seems possible.
Blue zircon was a particular favorite in Victorian times, when fine gems were often featured in English estate jewelry dating from the 1880s. Gemologist George Kunz—Tiffany’s famed gem buyer—was a notable zircon advocate. He once proposed the name “starlite” to promote the gem’s fiery nature. The name never caught on.

ZIRCON QUALITY FACTORS

The most valuable colors of zircon are blue, bright red, and green.
Zircon is often eye-clean. Gems with noticeable inclusions are less valuable.
To maximize its brilliance, zircon is most often cut in rounds and ovals.
Zircon in fine quality is rare in large sizes. Zircon weighs more than most gems of like size.
Zircon is a gemstone that’s not commonly known among jewelry buyers, which is a shame considering the number of beautiful colors it comes in. These include earth tones such as cinnamon, sherry, yellow, orange, and red. Among those who are familiar with this gem, zircon is especially admired for its attractive blue colors.

Color

Some zircons display warm autumnal earth tones such as yellowish and reddish brown, inspiring fashion trends. Red and green zircons have market value as collectors’ stones, and cat’s-eye zircons occasionally appear on the market. There are also colorless zircons.
Although collectors clearly love zircon’s color variety, consumers seem most enamored of just one hue: blue. Gem dealer reports indicate that at least 80 percent of zircons sold are blue.
Because they’re in greater demand, blue zircons usually command higher prices than any of the other varieties. Even though gem buyers can satisfy their demand for blue gems with top-grade topaz at significantly lower cost, blue zircon continues to sell well. Industry analysts believe that blue zircon has yet to reach its full market potential.
Zircon’s blue, almost always the result of heat treatment, comes in a range that includes very slightly greenish blue, greenish blue, and very strongly greenish blue.

Clarity

Zircons are relatively free of inclusions, but many untreated zircons have a cloudy or smoky appearance. If it’s extreme, it can be a negative factor with buyers. In Victorian times, this smokiness made zircon a popular gem for mourning jewelry.
Today, most zircon that is faceted for use in jewelry is free of inclusions that are visible to the eye. Eye-visible inclusions cause a drop in zircon value.
Rarely, zircon might contain long parallel inclusions that create the cat’seye effect when the stone is cut as a cabochon.

Cut

It’s a challenge to cut zircon because the gem is brittle. Cutters usually fashion zircon in the brilliant style to take advantage of its luster and fire. A modification of the brilliant cut, known as the “zircon cut,” uses eight extra facets around the gem’s lower portion, called the pavilion. This isn’t seen very often today because of the extra labor costs involved. Zircon can also be found in step cuts, which have rows of parallel facets, and mixed cuts, which are a combination of brilliant and step-cut facets.

Size and Weight

The supply of zircon is generally limited, and typical sizes depend on color. Blue or green stones normally range from 1 carat to 10 carats and yellows and oranges up to around 5 carats. Reds and purples are usually smaller.

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