The quality and value of any stone is evaluated on certain parameters.
Origin - Naturally occurring Star Sapphires are very rare and are predominantly found in Sri Lanka. Star Sapphires have also been found in Burma, Cambodia and India but Sapphires from the Ceylon Sri Lankan region display a well-defined star effect.
Colour - Star Sapphire is found in various shades of blue, grey, pink, purple, brown, green, yellow and orange in nature. Blue Star Sapphires are the most common while orange and yellow are very uncommon. Colour Changing Star Sapphires also exist but are exceptionally rare. The value of Star Sapphires is affected by the intensity and attractiveness of the body colour along with the strength and sharpness of the star.
Clarity - Star Sapphires have rutile silk inclusions that cause the star phenomenon in them. An ideal Star Sapphire would have enough silk for a well-defined star without compromising the stone’s transparency but Star sapphires rarely have the combination of a fine translucent or transparent color and a sharply defined star.
Cut - Star Sapphires are cut in smooth domed cabochon shape to bring out the star effect. Cuts which have a bright properly oriented star are highly rare and valuable.