Holts London Guide To: Phenomenal Gemstones

Posted on the 24th March 2014

Phenomenal Gemstones: Gemstones that have special optic characteristics created by their unique structure, when seen under different lighting or viewing directions are known as ‘Phenomenal Gemstones’. Most common phenomena: Iridescence (Labradorescence, Play-of-colour) Chatoyancy (Cat's eye, Asterism) Color-change Adularescence Aventurescence Gemstones that are considered Iridescent show a multicolour effect created by their inner structure where white light is separated into spectral colors…it’s like a rainbow in/on the stone! The basic subcategories of Iridescence are: Orient, Play-of-colour and Labradorescence. The most familiar phenomenon to everyone is Orient because it is the phenomenon of pearls, shells and mother-of-pearls. It is that silky looking sheen that sometimes has overtone colours, for example a white pearl with a red overtone. Another very recognizable subcategory of Iridescence is Play-of-Colour, the phenomenon of the magical Opals! It describes the colourful patterns seen in an opal created by its unique internal structure of stacked spheres. Labradorescence is the phenomenon that describes the very characteristic type of Iridescence that is found in Labradorites; it is a neon blue, green and whitish sheen that moves around as you move the stone! Chatoyancy is a phenomenon created from well oriented inclusions that are called needles, because of their resemblance to them. The most common type of Chatoyancy is Cat’s eye, where a bright straight line appears on the stone and it moves as you tilt the stone under a light source. The phenomenon is best created on domed stones (i.e. cabochon) and the most representative stone that can be found everywhere, from high-end gemstone stores to flea markets all over the world is Tiger’s eye Quartz! Asterism is an even more type of Chatoyancy, in which the needles that create the phenomenon are intersecting and therefore instead of a line they create a moving star on the dome of the stone. The most representative examples of this phenomenon are Star-Rubies and Star-Sapphires. Adularescence is the characteristic phenomenon of moonstones; it depicts the mystical milky sheen that is visible on the surface of the stone, in most of its colour variations. The most valuable type is the blue sheen moonstone. When you look at stone and you see “glitter” everywhere you know you are looking at Aventurescence! Best representatives of this phenomenon are Sunstone Labradorite, Sunstone Oligoclase and Aventurine Quartz. The first two range between orangy pink and red and Aventurine Quartz has a translucent to opaque green colour. Some of the most valuable gems display the Color-Change Phenomenon. One of the rarest gems in this category is Alexandrite, a variety of Chrysoberyl. The finest type of this gem is purplish red under incandescent light and bluish green under daylight or fluorescent light. It is like having an emerald at work and a ruby at dinner…in one gem!