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Heat Treatments on Gems

  1. Apr 29, 2008 #1

    I am a student of gemology and just can't find a good explanation for the new "Glass Filling" treatment on Rubies, especially from Madagascar.

    The following is a short description of how the treatment is done;

    1) Ruby rough is dipped in Hydrofluoric acid for about 48 hours. This creates some holes, like very precise drills on the gem which helps to increase the clarity too.

    2) The gem is placed in a heating oven for about 5 to 10 hours with an average temperature of 1800C.

    3) During the heating process, Silicon powder is added all through the gems, which are somehow part of the ruby when its cooled down.

    the silicon makes the very low quality rubies good for even jewelers thats why its becoming more & more popular everyday, although the treatment is not acceptable!!!

    This Ruby has a massive practical difference compared to classical heating... It cant undergo electrolysis. If it is passed through electrolysis the stone is somehow cracked from the surface which makes it useless for jewelery.

    Why should this cracking happen?

    Any Help will be highly appreciated.
  2. jcsd
  3. Apr 29, 2008 #2


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    This link will get you started:


    Heat-treating many gems is acceptable to the gem market and is usually done to improve color. For example, Tanzanite rough is trichroic, but one of the three colors is a not-too attractive orange/brown. The market accepts heat-treating Tanzanite because it results in attractive dichroic stones with blue and purple colors. In this case you mention, the heating is done to fuse the fractures in the low-grade rubies so they will be clear enough to refract light when they are faceted. The fractures are filled with foreign materials that have a refractive index closer to that of corundum so that the filled cracks are less visible.
  4. Apr 30, 2008 #3
    Many Thanks for your informative link.

    Yes you are right heating any gem is acceptable but as long as its considered as simple heating (adding nothing).

    In gems treatment capital (Thailand) bulk diffusion gems are also considered as Heated and the vendors wouldn't say anything about it!!! Red Sapphires are an example... Sapphires just cant be red, due to low level of chromium. Beryllium is used for changing the color of white & pale violet sapphires from Songea (Tanzania) & Madagascar.

    As you have mentioned gems are treated for a better look but thats just the good side of the story!!! The additives used are far more worse during heating stage; syonide, beryllium and many other highly toxic materials are being used thats why its not always acceptable!! The classic heating is 100% okey.

    The silicon causes the gem to break after passing an electric current, why is this?
  5. Apr 30, 2008 #4


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    I don't know, but I suspect that it is because of a differential in the thermal expansion rate of the gem and the material used to fill the voids in the gem. If one material is more resistive than another, it might heat up more when you try to run a current through it. No filled/fused stone is going to be as stable and durable as one that is pure and solid.
  6. May 4, 2008 #5
    Thats a very likely statement...

    Thanks for that :) You have been a great help :)
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