Tuesday, December 24, 2013

Star Sapphire

A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism; red stones are known as "star rubies". Star sapphires contain intersecting needle-like inclusions following the underlying crystal structure that cause the appearance of a six-rayed "star"-shaped pattern when viewed with a single overhead light source. The inclusion is often the mineral rutile, a mineral composed primarily of titanium dioxide.

The value of a star sapphire depends not only on the weight of the stone, but also the body color, visibility, and intensity of the asterism.
Star Sapphire is usually found in blue colors, but there are also various shades of brown and green that are called black star sapphire. Orange and yellow star sapphires are almost unknown, and very rare. Color changing star sapphires are even more of a rarity.

The coloring agents in blue sapphire are iron and titanium and, in violet stones, vanadium. A small iron content only results in yellow and green tones, chromium produces pink, iron and vanadium orange tones.
The most desirable color is a vivid, intense blue.

Less transparent sapphires, translucent or opaque stones, are cut en cabochon to support the star effect with its six rays. The best cabochons are somewhat transparent, with smooth domes of good symmetry.
The Star of India is a 563.35-carat star sapphire, one of the largest such gems in the world. It is almost flawless and is unusual in that it has stars on both sides of the stone. The greyish blue gem was mined in Sri Lanka and is housed in the American Museum of Natural History in New York City.
The Black Star of Queensland is a 733-carat black sapphire, and the world's largest gem quality star sapphire. It was discovered in Australia in the 1930s.