By Peter G. Read
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Extra info for Beginner's Guide to Gemmology
Diamond CUBE Fluorite Pyrite OCTAHEDRON <^ Diamond Spinel Diamond RHOMBIC DOOECAHEORON Garnet Pyrite Diamond ICOSI TETRAHEDRON Garnet Fig. 3. The cubic system, with examples of pyrite (rear), spinel (left) and fluorspar crystals CRYSTALLOGRAPHY 41 ircon Fig. 4. 3). ) (2) The tetragonal system This has t w o equal-length axes at right-angles t o e a c h o t h e r , and a third (principal) axis which is either s h o r t e r or longer t h a n t h e o t h e r t w o , a n d at right-angles t o t h e m .
2) The tetragonal system This has t w o equal-length axes at right-angles t o e a c h o t h e r , and a third (principal) axis which is either s h o r t e r or longer t h a n t h e o t h e r t w o , a n d at right-angles t o t h e m . T h e r e are five axes of s y m m e t r y (four two-fold a n d o n e four-fold), five planes of s y m m e t r y a n d a c e n t r e of s y m m e t r y . 4). (3) The hexagonal system There are four axes in this s y s t e m , t h e first t h r e e being of equal length and intersecting each o t h e r at 6 0 ° in t h e same plane.
Those materials which have been stained simply t o i m p r o v e their colour can often be d e t e c t e d b y fluorescent or spectroscopic methods. As already m e n t i o n e d , d i a m o n d s can be artificially coloured by irradiating t h e m with n e u t r o n s in an a t o m i c r e a c t o r . This p r o d u c e s a h o m o g e n e o u s green b o d y colour, which can be changed t o yellow or a c i n n a m o n b r o w n by a s u b s e q u e n t h e a t t r e a t m e n t at 5 0 0 - 8 0 0 ° C .
Beginner's Guide to Gemmology by Peter G. Read