Experts refer to the ‘silk’ of a ruby when describing the wonderful silky shine of the red corundum. This phenomenon is due to extremely fine needles of rutile, a mineral which forms a star-shaped deposit within the ruby and causes a captivating light effect technically referred to as ‘asterism’.
If rubies containing rutile are cut as semi-domed cabochons, the result would be a six-point star, which seems to glide magically across the surface of the stone when the latter is moved. Star rubies are precious rarities.
While their value is dependent upon the beauty and attractiveness of their color, the nature and position of the rays within the stone is also an influential factor. The most valuable star-rubies display fully formed rays along an imaginary line dissecting the stone, while the star itself is situated at the very center of the ruby.