Cat's eye

A cat's eye like effect, known as 'chatoyancy', appears to move on this stone's surface. Cat's eye is a gem variety of chrysoberyl. 

Hardness: 8.5 on the Mohs' scale.

There are generally two varieties of cat?s eye ? the alexandrite cat?s-eye and the chrysoberyl cat?s-eye, which is very popular in the Far East, particularly in Japan. The ideal colours of the chrysoberyl cat?s-eye are yellowish-brown, which is called the honey colour, and the yellow-green, which is called the apple green colour. A very good cat?s eye, apart from being of ideal colour, should have a high degree of transparency and a well-defined unbroken ray. It should be free from any distracting inclusions visible to the unaided eye. The chrysoberyl cat?s-eye is one of the most beautiful gemstones because of the ?chatoyancy? or the eye effect. 

Description 
A translucent variety of chrysoberyl (beryllium aluminum oxide) which exhibits a silvery white line across the stone. This moves as the stone, the light source or the observer moves and may appear to open and close like an eye. The finest quality has a sharp eye that appears to open and close as the stone is rotated, and exhibits a strong "milk and honey" effect (stone on one side of the eye appears lighter than the other). These colours switch as the stone or light source is moved. The most highly prized body colours are greenish-yellow and brownish-yellow (honey colour). 

Varieties 
Rare specimens also exhibit change of colour. 

Sources Sri Lanka, Brazil. 

Phenomena 
Chatoyancy caused by the reflection of light off minute, parallel, needle-like rutile crystals or hollow tubes. 

Toughness 
Excellent 

Miscellaneous 
When a gem specimen exhibits both chatoyancy and change of colour, one or both phenomena will suffer. It is more common to find a good eye with poor change of colour. The conditions necessary for one phenomenon conflict with those needed for the other. The term cat's eye when used alone refers to chrysoberyl. Other minerals exhibiting chatoyancy must be qualified, e.g. tourmaline cat's eye. 

History 
Cat's eye has been regarded as a preserver of good fortune. The natives of Sri Lanka still consider it a charm against evil spirits. British royalty often use it as an engagement stone. 

Cuts & Uses 
Must be cut in a cabochon to produce cat's eye effect. This should be cut so that the long portion of the cabochon is 90 degrees to the direction of the needles.