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933 Views 7 Replies Last post: Nov 27, 2012 8:41 PM by brettarcher RSS
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Nov 26, 2012 11:13 PM

small crystal bought in Kenya roadside ne Kilmanjaro

hi ya all

 

As we say in Texaz although i live in London ...

Bought this small pepple gem opaque semi - with small (v small patch of some blue crystal )

 

from a road side seller in foothill s of Kilamanjaro kenya

it scratches glass ..

no real crystal shapes ....

possible ???

  • golden beryl
  • topaz
  • corundum

 

with aquamarine or sapphire ????

 

these pics are magnified x10 at least ......

 

Any ideas how i can further id ?

how do i do Spec Gr with basic home stuff or Refractive  Id ???

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  • As it looks quite "quartzy" (but isn't quartz!), could it be: Zoisite? Often yellow, but with a bluish variant called "tanzanite" found in the foothills of Kilimanjaro (which by the way lies in Tanzania, if they haven't moved it since I saw it...). It is somewhat harder than glass.

     

    To decide SG you at least need a good scale that is able to weigh with a few figures (at least two...) in the range considered, and a vessel in which your object just fits (the closer the fit, the better the result!). Then weigh (in grams): 1) your object (only the object - not in the vessel!) 2) the vessel filled with water to the rim (or better a mark1) 3) the vessel with your object in it and then filled to the rim (or the same mark) with water. Calculate [1] / ([1] + [2] - [3]) and you get the SG in g/cm3 (where [1] means the result of weighing number 1, etc.).

     

    The refractive index is so hard to measure with only "kitchen equipment" that I consider it impossible.

     

    1 the mark should be at a height so that the object will be fully covered.

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    • On this page there is both yellow and blue tanzanite/zoisite from Merelani.

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      • The "cheapest" (and best) scales are found at labs - like schools, hospitals - and they also have a good range of suitable vessels. Perhaps you know a science teacher, or somebody who knows a science teacher... If you have left school, not only will I think that your old teacher (if still alive, of course) would like to hear from you, I also think that he/she will be able to help you and teach you how to do good measurements.

         

        Today there also are electronic "letter scales" that have quite a good range, and are quite cheap, but whether they are any good and useful I don't know...

         

        New high quality lab scales are not cheap (in the magnitude £1500 and, of course, infinitely upwards), but you can do with simpler scales, as on the other hand is your precision of filling the vessel to exactly the same level twice (a height difference of 1 mm in a beaker with a bottom area of 1 dm2 is 10 g - no use in being able to weigh with a precision of 0.01 g then...)

         

        You can do some estimations. Let us say that your object is a cube 5x5x5 cm with SG 3.000 g/cm3. It then weighs 375 g (weighing 1). Your vessel has a base area of 50 cm2 (twice of the object) and weighs 100 g. You fill it to 6 cm with water - i.e. 300 g of water (thus weighing 2 gives 400 g). Weighing 3: object (375 g) + vessel (100 g) + water (175 g) gives 650 g. Calculate 375 / (375+400-650) = 375/125 = 3 (our figures are correct!). Now if you make an error of 1 mm when you fill the vessel for weighing 2 and 3 this means an error of 5 g - but in reality you should be able to be somewhat more precise1, let's say 0,4 mm = 2 g and add to that 1 g in error of the scale: Weighing 1: 375+/-1, weighing 2: 400+/-3, weighing 3: 650+/-3. Now you worst outcomes (note that the error in weighing 1 adds or subtracts the same way both in the nominator and denominator!) are 374/130 (=2.877) and 376/120 (=3.133). With a perfect scale for £1500+ you would have the limits  375/129 (=2.907) and 375/121 (=3.099). I.e. the differences you get in SG in the worst (or is it best?) case with a cheap £20? 1-g-"precision" scale and a 10-mg-precision scale for £1500+ is 0.034. This is about equal to a difference of about 0.1 mm error in filling the vessel.

         

        Of course there also are special devices for measuring density (=SG) that works on the buoyancy-principle, but then you are quite far from "ordinary kitchenware".

         

        It is quite reasonable to assume that it is zoisite/tanzanite and that the locals find it in waste from the mine. There is also the possibility that workers steal and smuggle out gemstones, but in this case the quality wasn't that high (and stolen high quality stones are not sold openly in the wayside, but in dark alleys).

         

        1 Especially if you fix an object as a "standard" to the vessel - like a needle pointing downwards a bit off the wall of the vessel - which the water will reach when you fill it carefully you can get even better precision. If the "needle" is made of glass the water will jump up on it when it reaches it which is easily seen. Such arrangements of course are very sensible to rough handling, time-consuming and tiring. Also by repeating the measurements all the way from the start over and over and over again you will get averages that gives better and better precision.

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