Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III. THE PHYSICAL PROPERTIES OF GEM STONES. I. The most important group of physical properties we have to deal with is that dependent ou light. When light acts on a gem it may— A. Be reflected back again. B. Be transmitted. C. Produce phosphorescence. A. When light is reflected from a mineral two phenomena may be observed— a. Colour. b. Lustre. a. White light falling on a mineral and suffering reflection may reveal a certain colour of the stone. This is due to certain of the components of the white light being held back or absorbed by the substance, while others of the coloured rays are returned to the eye and there produce a sensation of colour. Should a stone reflect all the rays of white light in the same proportion as it receives them it will appear white ; if all or nearly all the rays are stopped it appears black; if all but the green rays are stopped we should say the stone was green, and so on. It is very unsafe to place much reliance on colour in the identification of precious stones, as in many cases one mineral may occur in several different colours, and on the other hand stones ofvery various kinds have unfortunately become known by the same name when they are of one colour. For example, a red Corundum is Ruby, while a blue crystal of the same mineral is a Sapphire, but a red Spinel may also be sold as a Ruby ; an expert eye can usually distinguish a difference in such a case, but it is not absolutely reliable. b. Lustre is of several different kinds, usually described as— 1. Metallic. 2. Adamantine. 3. Vitreous. 4. Greasy. 5. Resinous. 6. Silky. 7. Pearly. The lustre may be of the varying degrees of splendent, shining, glistening or glimmering. By far the most important in the crystallised gem stones is th...