Star rubies and sapphires show a bright six-pronged star (termed asterism), and are always cut in the shape of a cabochon. Furthermore, the back (or bottom) of a star gem is left unpolished so that light is trapped inside the gem. The stars are formed by the light reflecting off thousands of perfectly aligned tiny inclusions in the stone itself (termed silk or rutile). The value of star sapphires and rubies are determined by two main factors: the intensity and attractiveness of the body color, and the strength and sharpness of the star. All six prongs should be straight and equally prominent. The complete star should be centered in the middle of the gem, but should also be able to glide effortlessly around the cabochon with minimal direct light. Star sapphires and rubies rarely have the combination of a fine translucent or transparent color and a sharp prominent star. Star sapphires and rubies are becoming more rare as most rough today is cut into faceted stones after heating instead of being cut into cabochons to display their possible stars. Thus all star gems are very collectible as a result, whether they are sapphires or rubies.
To get an idea of the rarity of these gems understand that out of 100 rough sapphires and rubies mined only 3 will have stars apparent after being cut into the cabochon shape. Of these 3, one will have a poor color but a good star, one will have a poor star but a good color, and only 1/100 will have both a good star and a good color! Lastly, make sure that if you purchase a star gem that the seller mentions that it is a "natural" or "genuine" star sapphire or star ruby because most of the star gems on the market today are artificial (sometimes termed Lind, Linde, or Lindi): A recent introduction to the market are diffusion star gems where a star is artificially added to a sapphire or a ruby. If you do find a natural gem with a stunning star then buy it and keep it as an investment piece because these gems are becoming rarer by the day. Only celebrities could own these gems in the past (Mary Pickford, Joan Crawford, and Jean Harlow), but now with the help of Ebay you can purchase a nice specimen for you and your family!
I hope this was an informative and interesting read! -Eric Varma, from Ebay Store Estate Rings