A distinct six-rayed star appears when light strikes this gemstone's surface.
Tiny, needle-like "silk" inclusions cause the star to appear.
Some rubies exhibit a six-rayed star when light is shined on them, an effect known as "asterism." The star, which appears on the surface of a cabochon-cut ruby, will move when the gem is rocked. The star is not fixed on or below the surface of the stone—as the stone is moved the rays become thicker or doubled, and its character changes in a variety of ways.

The interior of a star ruby contains many "silk"inclusions. Needle-like crystals of rutile (titanium oxide) called "silk" cause the star to appear, and when light strikes these rutile inclusions, which intersect at 60angles to each other, a star appears on the surface of the cabochon. These silk inclusions also cause star rubies to be semi-transparent. The star ruby as shown in the photograph on the next page has a strong red color and strikes a precarious balance between high transparency and the visibility of its star, making it a miraculous gem-quality stone.

Corundum (the mineral name of ruby and sapphire) that is red and highly transparent is polished into faceted ruby.
Heavily included stones that are not expected to have an attractive appearance if faceted are generally made into cabochons, which emphasize their color. When a cutter comes across a semi-transparent to translucent stone containing silk inclusions, it is fashioned as a star ruby. Star rubies with a distinct red color are especially valuable and rare.

In Sri Lanka, time-tested methods are employed to heat stones to approximately 1,000C (1,850F). This treatment removes a characteristic bluish color from the rubies. This does not cause the red color to get darker, but when purplish star rubies are heated they often become a more pure red. However, when temperatures exceed 1,300C (2,370F), the silk inclusions that cause the star are altered, causing the star to disappear. Also, any liquid inclusions that are present can rupture during heating and cause the rough to fracture, making it useless as a gemstone. Deciding whether or not to heat a stone requires very specialized knowledge and experience.
Ring, Platinum
Star Ruby 1 pc
3.30 ct
Diamond 8 pc
US $30,000