Star sapphire is corundum that when cut en cabochon demonstrate a phenomenon known as asterism or star. The asterism is associated with intersecting needle like inclusion most commonly composed of the mineral rutile or hematite. The rutile needles are aligned so that they intersect each other at a difference of 60 degrees which in turn produces a beautiful star. Star sapphire most commonly exhibit a star with 6 rays or legs however in rare circumstances 12 rayed stars are known to exist. The 6 extra ‘rays’ within a 12 ray star is due to the presence of hematite along with rutile.
When the gem is held is direct sun light or under a single point sourced light, a star forms on the surface of the gem and seems to float over the surface as the gem is moved around. The star effect is caused by light striking the rutile needles and being reflected from them creating a line of light on the surface of the cabochon. The 3 sets of rutile oriented at 120 degrees difference create 3 lines on the top of the cabochon. These 3 lines intersect at the centre of the gem which inturn appears like a six rayed star. The quality of the star effect is determined by the amount of rutile present and how tightly the rutile is packed. The more tightly packed the finer the star will be.
In some cases, such as Black Star Sapphires, besides Rutile there are inclusions of Hematite in the form of plates which also forms three sets. If the rutile needles are present in large quantity, this causes a decrease in transparency of the stone. Hence why a lot of star sapphires are opaque to translucent however it is possible but rare to find them transparent. Rarity also increases with size just like any other gemstone!