What are star rubies?

Star Rubies are a rare variety of the gemstone Ruby. These magnificent gems 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 ruby is seen in a single light source such as sunlight and spotlight.

what causes the star-effect in star rubies?

Star-effect or "asterism" is caused by the dense inclusions of tiny fibers 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 rubies, they are never completely transparent. In fact, star ruby is one of those few gemstones which actually require inclusions (i.e. rutile).

Are star rubies rare?

Today, out of every 100 rough corundum (ruby and sapphire) mined; only three will have stars apparent after being cut into cabochons. Out of these three, one will have a poor color but a good star; one will have a poor star but a good color and only one out of hundred will have both a good star and a good color! Most original sources of star rubies such as Mogok (Burma) have run out of supplies. Worldwide production today is yielding a very small quantity of fine stars (especially in sizes above 5 carats). Star rubies are becoming rare as most rough today is cut into faceted stones after heating instead of being cut into cabochons to display the star. Till the 1960’s star rubies fetched more prices than faceted rubies!

 
Sources of star ruby

Most rough rubies that can be cut into stars are mined from Burma (Myanmar), Srilanka, India, Africa, Vietnam, Thailand, Afghanistan, Pakistan, Australia, Cambodia, United States. The most commonly available Star Rubies are usually from Africa and India.