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About Minerals

Star Stones

The optical phenomena of some gem materials to display a single ray of light on their surface is called chatoyancy, a French word meaning cat or cat's-eye. Gems displaying this characteristic exhibit a single undulating narrow band of white light with a changeable luster.

Another optical effect is shown when some gem materials exhibit more than one ray of light. These rays will cross or intersect each other at some central point or points on the surface of a cut and polished gem. This phenomena is called asterism or is more commonly known as a star.

The cause of asterism or chatoyancy is attributed to tubes, or needlelike inclusions within the gem. When these foreign inclusions are highly uniform in alignment within the gem, they will be capable of concentrating and reflecting or transmitting the light which enters the gem. However, this potential will not be effectual in the form of a ray or rays if the gem does not have the optical shape necessary for focus and magnification of the light. When the foreign inclusions are aligned only in one direction with the gem, a single ray of light will be possible. If the alignment is in two directions, then the gem will have the potential of emitting two rays of light which will intersect each other at a central point or points on the gem creating a star with four legs. When the alignment is in three directions, three intersecting rays can be emitted which will produce a six legged star.

Gem materials which are capable of displaying a ray or rays of concentrated light will usually show some indication of this phenomenon in the form of a satin sheen or silkish luster while in the rough state and when exposed to a incandescent type of light. The area in which the sheen or silk is most intense will usually yield a star or cat's-eye effect. This area should be tested with a testing fluid such as STP motor oil and marked prior to shaping. The gem is shaped so that this area will become the approximate apex of the gem' s dome or curved surface. Approximate is mentioned because the ray or rays will tend to shift their location slightly as shaping progresses. This shifting is attributed to the relationship between the gem's physical shape and optical properties. Any change in the physical shape of the gem will also exert a change in directions, focus and magnification of the ray or rays. Special care and star-making cups must be used when lapping the stones in order to achieve the desired effects. Read up on this technique before attempting to cut a star stone.

by Mary Prosek From The Opal, 10/07 via The RockCollector December, 2007