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By Peter G. Read
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Additional resources for Beginner's Guide to Gemmology
In natural blue d i a m o n d s , t h e colour-pro ducing defects are caused b y t h e presence of b o r o n a t o m s . While t h e value of a coloured g e m s t o n e is influenced b y t h e d e p t h and p u r i t y of its colour, t h e price of d i a m o n d is usually d e t e r m i n e d by t h e c o m p l e t e absence of colour. T h e e x c e p t i o n s are t h e so-called 'fancies', whose shades of yellow, red, p i n k , orange, b r o w n , green a n d blue are sufficiently attractive a n d rare t o give t h e m an e n h a n c e d value.
The trigonal system. The examples of quartz (left) and calcite crystals illustrate contrasting habits in the same system. The rhombohedron shape can also be arrived at by cleaving a hexagonal shaped prism of calcite 44 CRYSTALLOGRAPHY T h e bulk of trigonal crystals have w h a t is described as normal s y m m e t r y , and this consists of o n e axis of s y m m e t r y ( t h r e e fold), t h r e e planes of s y m m e t r y and a centre of s y m m e t r y . 6). ( E x a m p l e s : q u a r t z , c o r u n d u m , t o u r m a l i n e , t h e latter having no centre of s y m m e t r y .
A six-sided bi-pyramid with u n e q u a l sides. G r o w t h hnes on t h e surface of a crystal. A crystal having a flattened tablet-like h a b i t . T h e crystal form often f o u n d at t h e e n d of a prism. 1. T h e table includes t h e g e m s t o n e species c h a l c e d o n y , j a d e i t e , n e p h r i t e a n d t u r q u o i s e w h i c h are classified as crypto-crystalline. This t y p e of mineral has g r o w n n o t as a single large crystal b u t as an aggregate of small or microscopic crystals (or crystalline fibres) r a n d o m l y o r i e n t e d within t h e b o d y of t h e gem.
Beginner's Guide to Gemmology by Peter G. Read