Asterism
Definition
A star-like luminous effect seen in some crystals when light is reflected from inclusions of needle-like crystals (usually rutile).
Example
Star Sapphires and Star Rubies are prized for their perfect six-rayed asterism.
The Star Effect
Asterism is an optical phenomenon where a star-like figure of light appears on the surface of a gemstone when illuminated by a single light source. This is caused by the reflection of light from thousands of microscopic needle-like inclusions aligned in a specific pattern within the crystal structure. The most common inclusions are Rutile (titanium dioxide) needles.
4-Ray vs. 6-Ray Stars
- 6-Ray Stars: Found in Star Sapphires and Star Rubies. The rutile needles align in three directions at 60-degree angles, creating a six-pointed star.
- 4-Ray Stars: Found in Star Diopside and some Garnets. The inclusions align in two directions at 90 degrees.
To see the star, the stone must be cut as a smooth, rounded cabochon, not faceted.