December birthstones are Zircon, Tanzanite, and Turquoise

Zircon is a colorful gem with high refraction and fire. Colorless zircon is also 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 has many beautiful appearances. There are earth tones, cinnamon, sherry, yellow, orange, red and the famous zircon blue colors. Its 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.

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-value type. A zircon’s classification depends on its properties, which are directly related to the amount of radiation-induced damage done to its crystal structure. 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.

Many people have heard of zircon but never seen it. 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.


  • MINERAL:Zircon
  • COLOR:Blue, red, yellow, orange, brown, green
  • HIGH:925 to 1.984 (+/- 0.040)
  • MEDIUM:875 to 1.905 (+/- 0.030)
  • LOW:810 to 1.815 (+/-0.030)
  • BIREFRINGENCE:000 to 0.059 (low to high)
  • SPECIFIC GRAVITY:90 to 4.73
  • MOHS HARDNESS:6 to 7.5 (low to high)


Red and green zircons have market value as collectors’ stones, and cat’s-eye zircons occasionally come on the market as curios. Collectors love zircon’s variety of colors, consumers seem most enamored of just one—blue. Dealers report that at least 80 percent of zircons sold are blue.


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. Most zircon that is faceted for use in jewelry is free of inclusions visible to the eye. Zircons with eye visible inclusions will drop in value.


Cutters usually fashion zircon in the brilliant style to take advantage of its luster and fire. 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.


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.

(Gem Encyclopedia – Zircon, 2016)

Tanzanite – Lush blue velvet, rich royal purple; exotic Tanzanite is only found one place on earth, in Tanzania, near Kilimanjaro.

Tanzanite’s appearance is influenced greatly by its pleochroism, which is the ability of a gemstone to show different colors when viewed in different crystal directions. Tanzanite can be violet-blue, similar to a sapphire color, or much more purplish. Often, both colors are readily visible in a stone when it is gently rocked and tilted.

Tanzanite is relatively new to the colored stone galaxy. in 1967 a Masai tribesman stumbled upon a cluster of highly transparent, intense blue crystals weathering out of the earth in Merelani, an area of northern Tanzania. He alerted a local fortune hunter, who quickly registered four mining claims. The fortune hunter hoped he’d been shown a new sapphire deposit; however, the deposit contained one of the world’s newest gems.

Within a short time, 90 more claims appeared in the same 20-square-mile area. No one was quite sure what the beautiful crystals were, but everyone wanted to lay claim to the profits they were certain to produce.

Almost overnight, tanzanite was popular with leading jewelry designers and other gem professionals, as well as with customers who had an eye for beautiful and unusual gems. The instant popularity of this transparent blue to violet to purple gem was tied to its vivid color, high clarity, and potential for large cut stones.


COLOR A deep saturated blue is the most valuable color of tanzanite. Paler tones are more affordable. Untreated, tanzanite is typically brownish. Most blue tanzanites for sale today owe their color to heat treatment, which is what reveals its attractive blues and violets. The blue crystals originally discovered by Masai tribesmen were an exception because they’d probably been exposed to a natural heat source within the earth at some point.

CLARITY Eye-visible inclusions decrease the value of tanzanite, particularly in lighter colored stones.

CUT Tanzanite is available in a wide range of shapes but cushion and oval cuts are most common.

CARAT WEIGHT Tanzanite is available in a wide variety of sizes, shapes, and faceting designs. The finest and deepest colors are usually seen in sizes over 5 carats. Smaller stones are often less intense in color.


MINERAL: Zoisite


COLOR: Blue to violet to bluish purple

REFRACTIVE INDEX: 1.691 to 1.700

BIREFRINGENCE: 0.008 to 0.013



(Gem Encyclopedia – Tanzanite, 2016)