Browsing Archive: May, 2021

COLOUR-CHANGE GARNETS – THE USAMBARA EFFECT

Posted by Site Moderator Webmaster on Monday, May 24, 2021, In : Mineralogy 

Duncan Miller

There are several different mechanisms for garnets to show a change of colour. The most common of these is analogous to the colour change in the well-known alexandrite variety of chrysoberyl. This is due to differential transmission of different wavelengths of visible light, leading to a difference in perceived colour under lighting with different degrees of red or blue light. A less well-known type of colour change in gemstones is the so-called Usambara effect, named after t...


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COLOUR, ZONING AND FALSE COLOUR IMAGING IN GARNETS

Posted by Site Moderator Webmaster on Monday, May 24, 2021, In : Mineralogy 

Lesley Andrews

Garnets are not only attractive, but also useful. As residents of the Cape, many of us know about the use of garnets as markers in diamond exploration. Other examples include garnet use in sandpaper, especially for wood sanding, and the production of synthetic garnets for laser generation.

The colours of garnet group minerals and varieties is a complex subject. Not all garnets are red, in fact they are found in all colours except bright blue. Briefly, the most common ions inf...


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THE GARNET GROUP

Posted by Site Moderator Webmaster on Monday, May 24, 2021, In : Mineralogy 

by
Peter Rosewarne

This month we are checking out the garnet group of minerals, which most people will have come across in the form of jewellery, as mineral specimens and even in mundane articles such as sandpaper. The previous fluorite article started with some “C” words that apply to the mineral and in the same vein, the following apply to garnets, with some qualifiers; cubic, contain calcium (some), colourful (some), cleavage-free, conchoidal fracture, costly (some), contain chrome ...


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