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By Peter G. Read

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COLOUR, LUSTRE AND SHEEN 27 With t h e t r a n s p a r e n t a l i o c h r o m a t i c gem minerals beryl, c o r u n d u m , t o u r m a l i n e and t o p a z however, colour is m u c h less useful as a distinguishing feature, as these gems crystallise in m a n y different hues. In these s t o n e s , t h e colour d e p e n d s entirely on which of t h e transition e l e m e n t s was present at t h e t i m e t h e mineral was formed. In t h e case of t o u r m a l i n e , crystals are s o m e t i m e s found in which t h e colouring i m p u r i t y changed during t h e g r o w t h of t h e mineral t o p r o d u c e a prism which changes along its length from blue t o green or p i n k .

Perhaps t h e m o s t well k n o w n use of cleavage occurs w i t h d i a m o n d , where a large irregularly-shaped crystal is s o m e t i m e s p a r t e d by this m e t h o d r a t h e r t h a n b y t h e very slow alternative of sawing. T h e m o s t d r a m a t i c e x a m p l e of this occurred with t h e 3 1 0 6 carat CuUinan, which was initially cleaved i n t o t h r e e pieces. Before a d i a m o n d can be cleaved, however, a groove or ' k e r f m u s t be scratched in t h e surface of t h e d i a m o n d , parallel t o o n e of t h e four o c t a h e d r a l cleavage planes.

T h e r e are seven axes of s y m m e t r y (six two-fold and o n e six-fold), seven planes of s y m m e t r y a n d a centre of s y m m e t r y . 5). ( E x a m p l e s : beryl, a p a t i t e . ) Beryl Apatite Emerald t Fig. 5. The hexagonal system with ex­ amples of apatite (left) and emerald crystals CRYSTALLOGRAPHY 43 (4) The trigonal system This system also has four axes w h i c h are arranged in t h e same m a n n e r as in t h e h e x a g o n a l s y s t e m . T h e s y m m e t r y of t h e trigonal system is, however, l o w e r t h a n t h a t of t h e h e x a g o n a l s y s t e m .

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