DECEMBER BIRTHSTONES – ZIRCON is a colorful gem with high refraction and fire that’s unfairly confused with cubic zirconia.
Mineral: Zircon;
Chemistry: ZrSiO4;
Color: Blue, red, yellow, orange, brown, green;
Birefringence: 0.000 to 0.059 (low to high);
Specific gravity: 3.90 to 4.73;
Mohs harness: 6 to 7.5 (low to high);
COLOR: The most valuable colors of zircon are blue, bright red, and green;
CLARITY: Zircon is often eye-clean. Gems with noticeable inclusions are less valuable;
CUT: To maximize its brilliance, zircon is most often cut in rounds and ovals;
CARAT WEIGHT: Zircon in fine quality is rare in large sizes. Zircon weighs more than most gems of like size;
DECEMBER BIRTHSTONES – TANZANITE Poised between lush blue, vibrant violet, and rich purple, exotic tanzanite is found in only one place on earth, near majestic Mount Kilimanjaro.
Mineral: Zoisite;
Chemistry: Ca2Al3(SiO4)3(OH);
Color: Violet blue to bluish violet to violet purple;
Refractive index: 1.691 to 1.700;
Birefringence: 0.008 to 0.013;
Specific gravity: 3.35;
Mohs harness: 6 to 7;
COLOR: Deep saturated violet blue or blue violet are the most valuable tanzanite colors. Paler hues are more commonly found;
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 color is less saturated in smaller sizes. Gems must be above five carats in size to have fine color;
DECEMBER BIRTHSTONES – TURQUOISE Azure sky, robin’s egg blue: Vivid shades of turquoise define the color that’s named after this gem.
Mineral: Turquoise;
Chemistry: CuAl6(PO4)4 (OH)8.5H2O;
Color: Blue to green;
Refractive index: 1.610 to 1.650;
Birefringence: Not detectable;
Specific gravity: 2.76 (+0.14, -0.36);
Mohs harness: 5 to 6;
COLOR: The even blue color of this cabochon would be called Persian blue in the trade;
CLARITY: These free-form turquoise cabochons show a typical matrix pattern;
CUT: Although turquoise is usually cut into beads and cabochons, it can also be carved;
CARAT WEIGHT: Cutters work around large areas of matrix to yield pieces of evenly colored turquoise~
31 March 2019
