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 Post subject: Re: Light source for grading opal
PostPosted: Sun Apr 28, 2013 12:55 pm 
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Interesting. That could explain a lot, the one I saw that was just plain brown and chocolate-like. Certainly looked out of place.

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 Post subject: Re: Light source for grading opal
PostPosted: Sun Apr 28, 2013 3:50 pm 
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I suspect Scarodactyl's right about occasional mixing of opal from the two sources. Yet in a recent parcel of Welo I've noticed quite a bit of base color variation though none is the Shewa "chocolate" type.
I'll soon find out if it cuts differently than the usual Welo crystal of various colors.

Lee, your comments about matrix interest me. I have a lot of experience cutting Australian opal from Andamooka, Coober Pedy and Mintabie as well as Lightning Ridge and have never heard anything like that. Of course LR opal comes in many forms, from the well-known "nobbies" to seam types from various areas. I imagine there's some variation and opal is always full of surprises. But LR material has a long-standing reputation as one of the most stable of all Aussie opals. I'd love to chat with your friend about that.

Here's some shots of Welo base colors I noticed in my parcel:


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 Post subject: Re: Light source for grading opal
PostPosted: Sun Apr 28, 2013 4:13 pm 
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Those do all look like Welo to me. The base color from there definitely ranges quite a bit (from non-chocolate brown through greys, and occasionally black), and it does seem to correlate with troubles in cutting (though less so after cutting from what I've heard). The worst trouble our faceter had was with some dark crystal stones with brown streaks running through. They just fell apart, which was too bad because the color was amazing. So I can understand wanting to avoid buying dark rough on the basis of stability, though the final cut stones can be so darned phenomenal that I (as a less crafty nonprofessional) find myself really drawn to them despite the risk of disappointment.

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 Post subject: Re: Light source for grading opal
PostPosted: Mon Apr 29, 2013 1:59 am 
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Hi Rick, I am the friend Lee mentioned- great to know someone remembers my ramblings!

My friends in Lightning Ridge when selling used to avoid buyers with a liking for fluoro lights; it makes the opal look bad and undermines the sellers position.
The buyers of course, were well aware of this.

For selling blue on black with potch in the face, and overcast day is good!

Now, you would find it hard to buy a decent parcel at the Ridge.

In Australian jelly opal, amber to yellow should be avoided- if you look at it under 20x or higher you will note a myriad of suspended particles which I believe are responsible for the yellow to amber body colour.

This type of opal is often unstable and will craze after cutting. I imagine it is to do with the expansion coefficient and the weak bond between the denser precious opal and the suspended material that we used to refer to as "matrix".

It is just the same body colour as some of the highly unstable beer bottle brown and yellow welo material that cracks on the wheel, then crazes.

I never buy it unless it is water clear. I have not had one cut stone craze on me yet.

I notice the welo can often contain a lot of salt- enough to taste - which will wash out after repeated changes of water!
;-)

Matrix is a highly nebulous term and can mean different things from different fields- Queensland, Anadamooka, and the Ridge.

I was telling Lee about ridge material described above that seems so similar to the welo right down to the patternless homeless POC.
At the ridge- you often get "matrix" adhering to the back of a stone or inside the stone.

While i can't cite sources a number of academically inclined people hae told me that the porous potch we call matrix at the ridge forms in the same polygonal cubic faced columnar stack arrangement as teh precious opal but is much weaker.

Given the very different expansion coefficient of the two similar materials the proposal was that the "matrix" will pull the opal apart at a molecular level once formation stresses were released by cutting- then very quickly indeed!

while I am not a scientist, this seems consistent with logic and my observations over the years.


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 Post subject: Re: Light source for grading opal
PostPosted: Wed May 01, 2013 4:36 am 
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I do not agree regarding your opinon, and fear, that anything but water clear Welos are stable. I have both cut and rough pieces of Welo in many different shades, all seasoned for several years in dry Scandinavian environment. None has developed any cracks or crazing. I have had them soaked and dried out a couple of times during that period. What cracked (maybe 5 percent) did that after first soaking and dryeing.

Of course there are different qualities regarding stability in the Ethiopian material but do not discard what can be very valuable stonese just because of some color prejudice. I'd love to buy what you colorwise reject, if I can make a personal inspection and grading. I love the yellowish orange stones with high end colorplay and so do my customers.

Matrix is another story, that I like to avoid too in opals not necessarily for stability reasons but because it does not let the stones come to their top splendour.

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 Post subject: Re: Light source for grading opal
PostPosted: Fri May 03, 2013 12:39 am 
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Lee, I've cut and pasted from your earlier post:

"Colorless base like water is preferred, cloudy white is acceptable, light yellow is barely acceptable but a strong play of color makes it OK. Opaque in any color besides white, any orange base and any brown base is not acceptable"

Could you help assign values to the above categories?

For example, if colorless base like water is 10, would cloudy white be 8, light yellow 4 and orange/brown 2?

thanks!


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 Post subject: Re: Light source for grading opal
PostPosted: Fri May 03, 2013 12:06 pm 
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Hi Bill,
That system of numbers would be a major undertaking that I am not qualified to do on my own. You have a good start there, perhaps a brown would drop to being a 1. It sounds like Conny may rank an orange one way higher than James would and both are stone cutters. I am primarily a smith that has learned about gems because I needed to so cutting and gemology are more like secondary interest. I do buy and sell a lot of stones and do trust the water clear much more than the opaque and heavily included but that is just common sense more than experience. I have never had an Opal craze or crack so I lack the learning experience, however, I prefer to keep it that way. Best regards, Lee

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