How Does Asterism in Star Gemstones Work?

The Star Effect (also known as the “Asteric Effect”) is caused by reflections from long minute needle-like inclusions. These inclusions of foreign mineral sit in straight lines between the gemstone’s atomic lattices and run at geometrical angles to each other.
Long acicular rutile needles such as those found in Blue Sapphire are the “building blocks” of the Asteric Effect. When densely grouped together their mass reflects light to the eye in star like patterns.

Asterism is best viewed under direct light with singular light sources such light bulbs, direct sunlight, spotlights and penlights being the best sources of light to view this phenomenon. Diffused illumination such as in fluorescent strip lamps often renders the stars indistinct.
The value of Star Gemstones varies widely. When judging their quality and value, the following points should be taken into consideration:

1. The distinctiveness of the star
Is the star well contrasted against the body of the gem?

2. Length and straightness of each ray
Straight rays that extend from girdle-to-girdle are the most desirable and valuable.

3. Color
Bright and vivid coloration distributed evenly across the surface of the gem is most desirable. As with most gems, heavy banding, zoning and color patches are undesirable.

4. The cut and position of the star
When viewing the star, does it appear to sit on the top of the gem or slightly to one side?

5. The gem’s size and carat weight
Higher carat weights are rarer than smaller weights, commanding a higher premium per carat.