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Geohavens: World of Rocks, Minerals, Gems and Gemmology :: Gemstone Discussion :: Other Coloured Stones
 Classification of gems.
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| agianto |
Posted on 21-02-2009 20:54
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Junior Member Posts: 32 Joined: 10.02.09 |
Stone varieties have their specific name because of their unique composition, properties and/or characteristics. But it is sometimes very confusing on where to draw the line, for example: green beryl vs emerald pink or red toumaline vs rubellite blue tourmaline vs paraiba tourmaline pink sapphire vs ruby Because the naming has a great impact on the value of the stones, can somebody help to clarify this matter? Thank you. |
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| aloysiushan |
Posted on 23-02-2009 17:20
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Super Admin Posts: 41 Joined: 08.09.08 |
Indeed. Nomenclature is causing a big problem worldwide. In South East Asia and probably in Indonesia, there's a proliferation of "Alexandrite" lab reports for chrysoberyl that exhibit very weak colour change. Should that be called alexandrite? Paraiba and Paraiba-type tourmaline is also a recent problem that has caused spades of nightmare. The current stance on those two terms is: "Paraiba" is used only on copper bearing tourmalines from Paraiba, Brazil. Paraiba-type refers to copper bearing tourmalines. Some of these come from Nigerian and Mozambique deposits. Be careful though, there are big lawsuits going on about the use of those terms. Blue tourmaline that does not contain significant amounts of trace copper cannot be called Paraiba. These are usually the relatively darker steely blue tourmalines called indicolites. I must say that indicolites are desirable in their own way. Blues are probably among the rarer colours. Apart from that, rubies and pink/purple sapphires is also a perrenial headache; at the end of the day, I advise my clients whether they'd like the colour and would they call it a ruby. Even at a gem lab, things are not so certain. As far as I know, the problem of green beryl/emerald has been solved. Whether vanadium coloured or Chromium coloured, they are both called Emerald. Bluish-green beryl is coloured by Iron, and are typically heated to remove the greenish cast making it more desirable. Rubellite typically refers to a stronger pinkish-red tourmaline. Pink tourmaline I'd usually reserve for the sweet bubblegum pinks. Paprok Afghanistan produces beautiful sweet hot pinks; as do many African localities. Worse, colour perception is also strongly affected by the spectrum profile of incoming light. A stone in the shop can look completely different from under the sunlight. Or another country! I am sorry folks, colour is subjective. A seller will typically have a different opinion about colour from a buyer  If it is really borderline, seek an independent opinion of a gemmologist.
Edited by aloysiushan on 23-02-2009 17:27 |
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| agianto |
Posted on 08-04-2009 05:47
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Junior Member Posts: 32 Joined: 10.02.09 |
How do you define Alexandrite Chrysoberyl? According to IGS (http://www.gemsociety.org/info/igem13.htm), to be called Alexandrite Chrysoberyl, the colour change has to be from a greenish colour to a reddish colour. Other colour combinations are called Colour Change chrysoberyl. But in wikipedia (http://en.wikipedia.org/wiki/Chrysoberyl), "other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light." |
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| aloysiushan |
Posted on 09-04-2009 04:09
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Super Admin Posts: 41 Joined: 08.09.08 |
There is no international consensus on what level of colour change in chrysoberyl qualifies as Alexandrite. This has caused big problems in Singapore and as far as I am aware, in Indonesia as well. I am at the moment working on a research paper on that issue. Aloysius |
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| boblalux |
Posted on 23-04-2009 11:01
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Newbie Posts: 6 Joined: 21.04.09 |
What about cutting out the traditional names, as is sometimes done in the classification of rocks in geology? For example, the above case concerning the Paraiba tourmaline, one could call it 'Cu-bearing blue tourmaline from Brazil, or Nigeria' etc. A bit long-winded, but then why not? Indicolite and rubellite, as I understand it, are solely blue and a pinky tourmalines, so perhaps one could keep the terms, as well a schorl and dravite etc. In geology, one tends to use a rational system whereby (in a very crass example), one could call a granite, a quartz-felspar-plagioclase rock if one is not quite sure about the genesis of the rock. Bob |
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| aloysiushan |
Posted on 24-04-2009 15:20
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Super Admin Posts: 41 Joined: 08.09.08 |
Things in the trade is not as academics would like it. A lot of money hinges on nomenclature, and where gains can be made on naming things can be intentionally blurred. Alexandrites, rubies vs pink sapphires, etc are just 2 of numerous examples out there. Of course, calling rubies something like "Aluminium Oxide with Chromium chromophore" or "Red alumina" would be immediately unsellable. Nomenclature over treatment labels, or even with nacre thickness in cultured pearls are rather thorny issues as well. Academics like to make things very clear etc, but many times they fail to realise that they form a small proportion of the gems, jewellery, rocks and minerals trade. Money lies in the trade, and pink sapphires will be sold as rubies or very weakly changing chrysoberyl will be sold as alexandrites. Consensus is difficult, especially with colour involved. Academics can propose colour standards for alexandrites and rubies, but will the intended users - the trade - accept it? Who determines the standards? Who gave divine authority to gemmologists to curse some stones and let other stones through? It's a mess. At the end of the day, it boils down to how much a consumer knows, and willing to pay. At the same time of course, it is all salesmanship as well. Gemmology only goes this far. |
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| junyi |
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