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 Post subject: synthetic opal
PostPosted: Tue Jan 24, 2006 10:17 pm 
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Hi

This is probably a really stupid question but does anyone know if it's possible to synthesis matrix opal?


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PostPosted: Wed Jan 25, 2006 12:08 am 
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Location: The frozen north prairie :-/
It's being sold on eBay: http://media-for-you.com/products/Synth ... -d471.html


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 Post subject: synthetic opal
PostPosted: Wed Jan 25, 2006 10:27 am 
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Thanks for that morning dove. I thought that matrix opal had the matrix still attached but from the picture I'm clearly wrong so can someone please tell me what a matrix opal is and if/how it's different to boulder opal?

Thanks.


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PostPosted: Thu Jan 26, 2006 6:25 pm 
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Star Sapphire,

Matrix opal is formed within a porous stone - usually sandstone or limestone. This makes the opal and sandstone one rock. Andamooka in South Australia is very famous for this kind of Opal. It used to be considered worthless, as the matrix was usually very pale and chalky looking, so it was difficult to see the play of colour.

This was fixed by some clever opal miners, who devised a method of cooking the opal, first in a sugar solution for a couple of weeks, and then in hydrochloric acid. The idea of this is that the sugar would penetrate a couple of millimetres, and then the acid would burn the sugar away to leave just carbon, turning the whole thing black. The second cooking with Acid is the difficult bit, and can take from 5 minutes to 24 hours, and must be watched, as overcooking ruins the stones. (Although they can be polished and re-treated). With the black matrix absorbing light rather than reflecting it, it is easy to see the play of colour, thus making the stones much more beautiful.

The first stones to hit the market were sold as black opal, but it was not long before the treatment was discovered. This treatment is now accepted in the industry, stable and permanent. It is generally assumed that all Australian matrix opal has been treated in this way.

As for pricing, it generally now fetches higher than white and light Opal, and less than Crystal. However, colour is still the primary pricing consideration, so red matrix can be worth more that green crystal. (All other things being equal of course).

Matrix Opal is very underrated in my opinion, and a fraction of the price of black, with all the beauty. In Australia at leat, it is possible to pick up fairly large pieces at reasonable prices.

Having said all this, matrix opal can also form in ironstone, which is (as it name suggests) much harder. Ironstone cannot be treated in the same way as the Andamooka material, although I do believe that some dealers have experimented with carbon based dyes to darken the stone. Ironstone is a brick red colour usually, so shows a better play of than untreated Andamooka.

Like regular Opal, boulder opal forms in seams, however much of the Coober Pedy white opal we see, forms seams in very soft limestone, and it is easy to extract from these. Boulder Opal of course forms seams in tough rock, which are split to expose the opal seam. If the seam is thick enough it can be cut off, although often these make fantastic natural specimens.

So – after all my ramblings, boulder forms in a seam, matrix is absorbed into a porous rock! (Now why didn’t I say that to begin with?)

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PostPosted: Fri Jan 27, 2006 8:21 am 
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Martin wrote:
.., and then in hydrochloric acid. ..


Hi Martin,

I guess most probably for sulfuric acid.
because this acid can deshydrate sugar and turn it into carbon (black).
;-)

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Jean-Marie Arlabosse
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 Post subject: synthetic opal
PostPosted: Sat Jan 28, 2006 9:46 pm 
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Thanks Martin,

There was me thinking that stones with the matrix still attached were probably one of the least treated D'oh! How dumb am I?



P.S. Please don't any of you feel like you have to answer that question :wink:


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PostPosted: Sun Jan 29, 2006 5:33 pm 
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Gemca - thank you for that correction. Good job I havn't tried treating any myself, or I might have wondered what I was doing wrong....

Martin

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