Star sapphires are a unique variety of sapphires that exhibit a phenomenon called asterism. Asterism is the appearance of a star-like pattern on the surface of the gemstone when viewed under certain lighting conditions. Here's how star sapphires form:
- Mineral Inclusions: Star sapphires owe their distinctive phenomenon to the presence of needle-like inclusions of the mineral rutile (titanium dioxide) within the sapphire crystal. These inclusions are known as "silk" due to their appearance.
- Rutile Alignment: During the formation of the sapphire, rutile needles align in specific crystallographic directions within the gem. The intersecting needles form a three-fold or six-fold symmetry pattern within the sapphire, depending on the specific crystal structure.
- Light Reflection: When light enters the star sapphire, it encounters these aligned rutile inclusions. The inclusions act as tiny mirrors, reflecting the light in such a way that a star-like effect is created. The star appears as a luminous pattern with distinct rays, typically with six points (known as a six-rayed star) or occasionally with four points (known as a four-rayed star).
- Cabochon Cut: Star sapphires are usually cut en cabochon, which means they have a smooth, rounded top and a flat or slightly domed bottom. This cut is designed to maximize the visibility and symmetry of the star effect. The cabochon shape allows light to enter the gem and interact with the rutile inclusions, creating the star pattern on the surface.
The quality of the star phenomenon in a star sapphire is evaluated based on factors such as the sharpness, visibility, and symmetry of the star. The most desirable star sapphires exhibit a well-defined, centered, and evenly distributed star pattern that moves across the gem's surface as it is rotated.
Star sapphires are highly prized for their unique and captivating appearance. The intensity and visibility of the star, along with the quality of the underlying sapphire, influence their value in the market.