Star Light, Star Bright
Star rubies and sapphires show a bright six-legged star in the dome of their cabochon. The stars, like the eye of a cat's-eye, are formed by light reflecting off tiny inclusions in the stone. The value of star rubies and sapphires are influenced by two things: the intensity and attractiveness of the body color and the strength and sharpness of the star. All six legs should be straight and equally prominent. Star rubies rarely have the combination of a fine translucent or transparent color and a sharp prominent star. Star sapphires are more common but still very rare with fine color. Star rubies and sapphires are becoming more rare as most rough today is cut into faceted stones after heating instead of being cut into cabochons to display the star. Stars are very collectible as a result.
Color: Blue (All Hues), Black, Red (All Hues), Green, Yellow, Violet, Silver
Mohs’ Hardness: 9
Myanmar (Burma): A marvelous Burmese Star, the Star of Asia 330 carats; cabochon cut; blue-violet star sapphire; acquired in 1961 from Martin Ehrmann; once said to belong to the Maharaja of Jodhpur, now resides in the Smithsonian.
Thailand: With black star sapphires, the most valuable are the golden-star black stars from Chanthaburi, Thailand.
Sri Lanka (Ceylon): Deeper colors in Sri Lankan stones are mainly (but not always) found in the larger sizes, where the color builds due to the longer light paths. One of the world’s finest large example, the Rosser Reeves Star Ruby, was mined in Sri Lanka. The Star of India, at 563 carats, is the largest and most famous star sapphire in the world. Formed some 2 billion years ago, it was discovered, allegedly more than 300 years ago, in Sri Lanka. Industrialist and financier J. P. Morgan presented the sapphire to the American Museum of Natural History in 1900. Today, the Star of India and the Rosser Reeves Star Ruby resides in the Morgan-Tiffany Collection in the American Museum of Natural History in New York City.
Others: Gemstones can often surprise you, one of the finest star sapphires I ever saw was a blue star from Australia
Sapphire sometimes displays a three-ray, six-point star. These star sapphires are cut in a smooth domed cabochon cut to display the effect. The three intersecting rays of light have been said to represent faith, hope and destiny. The star is best viewed with a single light source. This effect, called asterism, is caused by light reflecting off tiny rutile needles, called "silk," which are oriented along the crystal faces.These fine, needle-like inclusions are what give sapphires their velvety quality. When these inclusions are numerous enough to make the stone translucent or opaque and are oriented properly, they allow light to be reflected in such a way that a star floats across the top of the stone with movement.
Tips on how to tell a real star from an artificial star
First thing to look at is the bottom, if there is an "L" stamped in the stone, it's a Lindy (or sometimes Linde) Star and synthetic... Lindy stars were manufactured en masse by the Lindy division of Union Carbide since about the early 1950’s, the Lindy division closed in the 1970's since then most artificial star sapphires are manufactured in Southeast Asia.
Next look for imperfections within the stone, or an unevenness on the bottom, or stripes or bands of color that shows through the top, most naturals have one or more of these natural imperfections, artificial stars gemnerally have an even color and smooth bottom.
Take a good look at the star itself, using a flashlight, most natural stars do not have a "perfect" star, whereas the natural will likely have one or more of the six legs uneven in length, or maybe not perfectly straight... the star must travel around following the light source, if the star is stationary it is definitely synthetic. The most common synthetics jump out as way are too perfect looking and the star might only move one direction if at all.
Judging a natural star sapphire
Color: Color is extremely important, top quality stars have top quality color, pretty simple
Star: The star should be complete and sharp, with no missing or broken legs, and each ray should extend to the girdle of the stone.
Clarity: Silk shouldn't not be concentrated so that it harms the transparency of the stone ( too much silk diffuses the light so it will affect color).Longer needles generally produce a better star than the tiny particles which are sometimes found
Transparency: The more transparent or translucent the stone the better. Tricky though, because sometimes with too much transparency the star is not evident.
Overall : Low-quality stars contain an over-abundance of silk . While this makes for a sharper star, transparency, and color usually suffer. A better transparency allows longer light paths, and richer color.