When viewed under the polariscope, this gemstone is doubly refractive. It has a birefringence of about 0.008 to 0.01 and has a uniaxial optical character it also has a negative optic sign. This is a gemstone with a transparency of semitransparent to opaque, and has a refractive index of about 1.762 to 1.77. The black star sapphire is a name that is used to cover the dark varieties of all the colors. The key separations of the star sapphire are appearance, the refractive index and through magnification. The star sapphire is a corundum by family. There are times when you find this gemstone in the yellow colors. If the gemstone is found in any other color apart from blue, then this color must be added to the name e.g.

The main ones are blue, black, brown, Purple, Green, Plum and rarely in orange.

The star sapphire comes in various colors. Usually, depending on the crystal, the star will have six to twelve rays of light. This is a very difficult effect to describe and it has to be seen in order to be believed. When there are two or more of the bands intersecting, then the star pattern is formed. The light will hit these inclusions, which are arranged parallel to one another and will force the light to be concentrated in points which form the star. The light that comes out of the star sapphire is caused by needlelike inclusions, which make the light to bounce around within the crystal and come out in the form of a star.

This effect would be lost if the stone was cut in any other fashion. Most start sapphire are cut in cabochon forms, and they have a dome that allows the asterism to be seen clearly. The stars which seem to appear closer to the top of the gemstone will have four points, but it is very rare that you come across one that has six points, and this is considered a truly valuable specimen. The depth and clarity of the gemstone will also affect this effect and also affects the price. Then position of the star effect, with respect to the positioning of the center of the gemstone will determine the value of the stone after cutting. This cannot be seen through the naked eye until it is cut. Introduction The star sapphire gemstone is a variety of sapphire which has been found to have a characteristic only known as asterism this is when the sapphire seems to produce a star of light that comes from the center of the stone If a skilled cutter takes a good look at the rough, he should be able to tell whether this will produce a start sapphire or not. Star Sapphire Gemstones - Variety of sapphire

Yellow star sapphire