Examining the Allure of Chatoyancy, or the Fascinating Feline Glow in Gemstones
In the captivating world of gemstones, one optical phenomenon stands out for its striking lustre and colour - chatoyancy. A recently published article in the Spring 2018 issue of Gems&Jewellery delves into this intriguing topic, focusing on sillimanite cabochons, a type of gemstone known for its fibrous structure.
Sillimanite, sometimes referred to as Fibrolite, exhibits chatoyancy due to its unique fibrous inclusions. When cut en cabochon, these fibres create a cat's eye effect, causing a band of light to hover above the surface of the stone, resulting in a mesmerising visual experience.
The article explains the difference between epi-chatoyancy and dia-chatoyancy. Epi-chatoyancy, which is predominant in sillimanite cabochons, arises from fibres or elongated inclusions aligned parallel to the surface of the gem. This alignment produces a sharp cat’s eye effect seen on the cabochon surface. On the other hand, dia-chatoyancy involves chatoyancy effects that occur within the body of the stone, caused by internal fibrous structures or inclusions oriented in different directions. This can create a more diffuse or multiple cat’s eye effects that do not rely solely on surface alignment.
The brown sillimanite from Orissa, India, is particularly known for its chatoyancy, attributed to ultrafine long and short ilmenite needles. The light path within the stone can be shorter in dia-chatoyancy, resulting in less absorption.
Drawing a ray diagram for chatoyancy is a challenging task due to the unknown location and orientation of the reflective surface within the stone. However, the solution lies in working backwards along the exit ray and forwards along the incoming ray, and then observing what happens where their paths cross if they are to be parts of a single ray.
The ray in the stone slopes up from horizontal at an angle of (r - i), which equals 12.4 degrees in the case illustrated. The base of the cabochon being fairly smooth helps more light from below to enter without being scattered, contributing to the visibility of the chatoyant effect.
In conclusion, understanding chatoyancy in sillimanite cabochons sheds light on the fascinating world of fibrous minerals and their optical phenomena. Whether observed from epi-chatoyancy (lit by the sun) or dia-chatoyancy (light coming up through the stone), the effects of this optical phenomenon are a testament to the beauty and complexity of gemstones.
- The published article in the Spring 2018 issue of Gems&Jewellery explores not only sillimanite cabochons and their chatoyancy, but also delves into the distinction between epi-chatoyancy and dia-chatoyancy.
- In addition to the published literature, workshops and courses on gemmology could provide valuable insight into understanding chatoyancy in various gemstones, including sillimanite.
- As the demand for education and expertise in the gemstone industry grows, many institutions offer certifications and memberships for those interested in studying gemmology and related fields, such as the science behind medical-conditions affecting gemstones and advancements in technology used in gemstone analysis.
- The Journal of Gemmology could be a great resource for finding more in-depth articles, publications, and research on chatoyancy, as well as other aspects of gemmology and the underlying science that governs Gemstone behavior.
