What causes the star-effect in star sapphires and rubies?

Star rubies and sapphires display a sharp six-rayed star which seems to glide magically across the surface of the gem when the latter is moved. This is caused due to an optical phenomenon known as "Asterism".

The star is best visible when the star sapphire is seen in a single light source such as sunlight and spotlight.

What causes the star-effect in star sapphires and rubies?

Star-effect or "asterism" is caused by the dense inclusions of tiny fibres of rutile (also known as "silk"). The stars are caused by the light reflecting from needle-like inclusions of rutile aligned perpendicular to the rays of the star. However, since rutile is always present in star sapphires they are never completely transparent. In fact, star sapphire and sapphire are one of those few gemstones which actually require inclusions (i.e. rutile).

Star sharpness

The star should be sharp (not blurry) and silvery / milky white. The best way to test the sharpness of the star is to look at the star sapphire in sunlight with the dome facing the sun. 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.

Clarity

Due to the presence of rutile (silk) star sapphires and rubies are never transparent. Most natural star sapphires found are translucent and opaque (mostly opaque).