Instructions
Hold the stone up to an extremely bright light to test its transparency. The slight haze from the asterism that produces the star means that all star rubies have some degree of opacity. The less opaque and the brighter the color, the more rare the star ruby. Consequently, finding a bright, sharply-defined star ruby is unlikely. If your stone fits the qualifications of an ideal star ruby, treat it with suspicion until further testing can be conducted. A completely transparent, clear star ruby does not exist.
Look at the top of the stone and examine the star. This effect is produced by the natural inclusion of slender rutile fibers in the stone. While extremely rarely a star ruby will feature a sharply defined, well-centered star, the majority of stars are off-center and hazy. If your stone contains a sharp, white star that is centered in the stone, and appears that it was almost drawn in the stone, you are looking at a fake. Real star rubies will be hazy around the star, the legs will not be equal in length and the star can be located in any part of the gem. Rotate the stone under the light. A true star ruby will feature a star that rotates with the light, while a fake star ruby will remain in one place.
Ask the vendor for the stone's certificate from a reputable gemology lab. True star rubies should be evaluated by a firm like the Gemological Institute of America or the American Gemological Society. The certificate will tell you the origin of the stone, whether or not it has been treated or altered and the specifications of that stone. If you need to ascertain the authenticity of an heirloom, both the GIA and the AGS can evaluate your stone at their laboratories. You can also consult with a local jeweler, who may have the equipment to evaluate your stone.