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Percentage of pigment
  
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Tom Mazanec




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PostPosted: Mar 14, 2018 07:29    Post subject: Percentage of pigment  

In a colored stone which is of a colorless mineral (like ruby or sapphire), how much of the material is the pigment? One part per thousand? A ppm? A ppb?
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James
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PostPosted: Mar 14, 2018 07:46    Post subject: Re: Percentage of pigment  

That may be a 'how long is a piece of string' type question
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Peter Lemkin




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PostPosted: Mar 14, 2018 08:39    Post subject: Re: Percentage of pigment  

Tom Mazanec wrote:
In a colored stone which is of a colorless mineral (like ruby or sapphire), how much of the material is the pigment? One part per thousand? A ppm? A ppb?

The question as worded shows you understand little of the MANY different causes of color in what you call otherwise colorless minerals. I'd say the answer would be from zero [and if you want to be fancy and also accurate, negative numbers] to 'lots' - depending on the mineral and the cause of color as we perceive it. I'd also never use the term 'pigment' - as that sounds like something you add to paint or similar. Some color is caused by spaces [absences of an atom or molecule in the matrix - in different patterns; some are caused by optical effects caused by minute [ppb, ppm] amounts of substances - often if not usually of no color or not of the color you then see [thus the reason not to use 'pigment']. Others are caused by polarizing effects, regular to semi-regular crystal defects or additional elements/molecules - and this is far, far from the full range of what causes colored stones and I was talking about natural colored stone. Radiation can cause [both natural and man-made] color change; heating can cause [both natural and man-made] color change and doping with colorants can cause man-made color changes. Again, not a complete list. A good chapter or color in gems or minerals would give a more complete explanation of this complex subject.
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Pete Modreski
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PostPosted: Mar 14, 2018 11:16    Post subject: Re: Percentage of pigment  

Tom, Peter Lemkin's reply to you was a good one.
But it's easier to give an answer for some minerals than for others. Those gem minerals that are colored by the simple presence of a metal ion that is a "chromophore", that has a distinct color, can be easily analyzed and a clear result provided Ruby is one such; in fact, there's an old paper (1931) available online, http://www.minsocam.org/ammin/AM16/AM16_34.pdf on chromium content in ruby, that reported between 0.10 to 0.25 weight percent Cr2O3 in ruby.Emerald is also typically colored by chromium in the tenths of a percent range. Gem minerals like peridot and tourmaline likewise contain fairly large amounts of metals that give them their color.
But minerals that are colored by trace impurities that create or are associated with lattice defect centers in minerals, occur at a much smaller range of impurity content in the mineral, yes, in the ppm range. This is the case for minerals like saphhire, diamond, topaz, quartz, and many more. It's different for every mineral, and the situation can be very complex to understand, as color is often related to the interaction between more than one impurity.
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kakov




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PostPosted: Mar 14, 2018 12:31    Post subject: Re: Percentage of pigment  

Even if the colour might seem the most “obvious” property of many minerals, i.e. the one our eyes detect immediately, by far in most cases it is not an inherent property of the given material. For identification purposes, usually the colour is a “curtain” you must see through. It is more reliable to look for the lustre, cleavage, small textures etc to guide you in the right direction.
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James
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PostPosted: Mar 15, 2018 03:57    Post subject: Re: Percentage of pigment  

I went to a great talk this week about how we can tell the color of fossilized creatures - surface features are key and they have nothing to do with pigments. So color can be caused by so many things, as Peter said
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Tom Mazanec




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PostPosted: Mar 15, 2018 05:54    Post subject: Re: Percentage of pigment  

James wrote:
I went to a great talk this week about how we can tell the color of fossilized creatures - surface features are key and they have nothing to do with pigments. So color can be caused by so many things, as Peter said


I recall reading of a certain Cambrian fossil which is always dark red. That's some durability!
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