How To Judge Asterism

Asterism may be perceived as multiple chatoyancy.

The Star
- Is it sharp and intense?
- The star should be complete with no missing or weak rays.
- The star itself should roll or travel easily across the surface.
- The intersecting rays should meet as near as possible at the center of the stone. If the stone is cut so that one
of the arms of the star cuts across the shortest axis of the cabochon; the rays are evenly spaced and pleasing
to the eye. In the gem trade, this is described as "Open Star". If a ray is oriented across the longest axis of an
oval stone, then the rays will appear to be less pleasing to the eye. This is described as "Closer Star".

Body Color
- Does the stone have a good body color?

Clarity
- The stone should be free from cracks and fissures. Surface reaching cracks and fissures are undesirable
forming traps for dirt, weakening the stone.

Transparency
- There is a very fine balance between transparency of the stone and definition and intensity of the star. Too
much silk or inclusions will induce translucent or even opaque body color, while too little silk or inclusions
may create ill-defined star.

Cutting
- Is the cabochon shape pleasing to the eye?
- A fairly high dome is necessary to focus the star to the best advantage. This is because the dome of the
cabochon acts as a condensing lens to concentrate the rays of the star. If the dome of the cabochon is cut
as shallow curve it is liable to possess a nearly flat spot at the top producing diffused star that will almost
invisible unless the observer is directly above the stone. A well-proportioned dome with a smooth and even
curve, lacking any flat spots, should produce a sharp star visible from most angles.
- The weight below the girdle should not exceed approximately 20% of the weight of the stone. Most colored
stones are purchased by weight so excess material beneath the girdle that does not contribute to the body
color or to the star is an unnecessary expense. Some native cut stones are practically spherical. Such stones
should be valued at the weight they would be if cut to correct proportions.
- In highly transparent gemstones a concave base can be a decided advantage as it will provide greater
brilliance, although it should not contribute to the weight excessively. Ideally the back of a translucent or
near opaque stone should be flat, smooth, but unpolished. If the gem is sufficiently transparent it will show
any polishing defects such as pits or cracks on the base.
- Test the symmetry by spinning the stone on a flat surface. If the stone wobbles or the star is seen to oscillate,
this indicates an unsymmetrical stone. Also look out for any chips or nicks at the girdle.
- The surface polish should be good with no pits, nicks or pieces of matrix to disturb the movement of the star.