Chatoyancy also known as the term ‘Cats Eye’ is used to illustrate an extraordinary optical phenomenon in certain gemstones. In this case the Sillimanite gemstone. When the effect is present it appears as a bright narrow slit similar to the pupils of a cat’s eye. The effect is causes by parallel fibrous needle -like inclusions found within the gem. These inclusions interfere with the passage of light throughout the stones and result reflects the light back to the view as a thin line across the surface of the gem. Sillimanite Cats Eye is an outstanding stone, which would make a great addition to anyone’s collection or to be set in a perfect piece of jewellery.