Beginner's Guide to Gemmology by Peter G. Read

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Gemstone Crystal system Habit tourmaline trigonal triangular prism (generally with rounded faces, heavily striated along length) zircon tetragonal four-sided prism with bipyramidal terminations in jewellery. Because of their micro-crystalline s t r u c t u r e , t h e surfaces of crypto-crystalline gems are slightly p o r o u s , a n d can be stained with various colours. Agates are often stained t o p r o d u c e pleasing and contrasting shades of this b a n d e d variety of c h a l c e d o n y , b u t pale specimens of j a d e i t e have also b e e n d y e d t o simulate t h e higher quality a n d therefore m o r e valuable green variety of this j a d e material.

T h e lustre seen with mother-of-pearl. A fibrous lustre typical of satin spar. These adjectives are only i n t e n d e d as relatively b r o a d de­ scriptions of t h e surface a p p e a r a n c e of a polished s t o n e , b u t in recent years an i n s t r u m e n t called a reflectivity meter has m a d e it possible t o measure a s t o n e ' s lustre with s o m e accuracy. Details of this measuring t e c h n i q u e , a n d t h e way in which it is used in g e m s t o n e identification, are given in C h a p t e r 7.

1) The cubic system Crystals in this system have t h e highest order of s y m m e t r y a n d are s o m e t i m e s called isometric. T h e cubic system has t h r e e axes, all of which are of equal length a n d intersect each o t h e r at right-angles. There are t h i r t e e n axes of s y m m e t r y (six t w o ­ fold, four three-fold and t h r e e four-fold), nine planes of s y m m e t r y and a centre of s y m m e t r y . Diamond CUBE Fluorite Pyrite OCTAHEDRON <^ Diamond Spinel Diamond RHOMBIC DOOECAHEORON Garnet Pyrite Diamond ICOSI TETRAHEDRON Garnet Fig.

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