By Peter G. Read
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Extra info for Beginner's Guide to Gemmology
Elements of symmetry So far, t h e adjectives 'regular', ' s y m m e t r i c a l ' and ' p a t t e r n ' have all been used t o describe a crystalline s u b s t a n c e . In crystal lography, t h e c o n c e p t of s y m m e t r y as applied t o t h e crystal structure is very i m p o r t a n t . F o r m o s t gemmological w o r k it is sufficient t o be able t o recognise t h e gem crystal h a b i t s , a n d t o be aware of their associated optical characteristics. However, CRYSTALLOGRAPHY 38 t h e three elements of symmetry are f u n d a m e n t a l t o t h e s t u d y of crystallography, and are defined here for t h e sake of completeness.
Often used t o describe a t y p e of inclusion (as in moss agate and green tree agate). This is a form whose faces intersect t h e vertical axis and o n e h o r i z o n t a l axis, b u t are parallel t o t h e third axis. This is a g r o u p of similar crystal faces. g. t h e garnet g r o u p ) . g. t h e garnets). g. graph ite and d i a m o n d ; andalusite, k y a n i t e and silHmanite). A crystalline s t r u c t u r e c o m p o s e d of straight or curved plates or leaves. R o u n d e d intersecting c o n t o u r s ( e .
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. F o r this reason, t h e h a b i t of crypto-crystalline minerals is always massive, a n d t h e gems are mainly semi-translucent or o p a q u e .