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 Post subject: Opal color grading
PostPosted: Fri Aug 12, 2016 11:13 am 
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Hi, I got this opal grading card from ebay. Is it valid for grading brown opals?

Thanks


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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 12, 2016 12:53 pm 
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Brown opals?
Can you post a picture of a brown opal?


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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 12, 2016 2:01 pm 
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I suspect he means the various flavors of chocolate opal that come out of Ethiopia--light to dark browns, grading towards black. The Shewa ones were the most notable (though many were unstable) but Welo produces some too.
Like this
Image
Or this
Image

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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 12, 2016 3:02 pm 
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I think the grading scale in question was developed for Australian opal, especially light to black opals from various Aussie fields, especially Lightning Ridge. While I've seen brownish "beer bottle" potch from Australia I can't recall any brown precious opals from there.

Since Ethiopian precious opal has some distinct differences in body color and structure from Australian it seems to be developing a grading system of its own. I don't think your scale is valid for Ethiopian "chocolate" body colors.

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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 12, 2016 4:24 pm 
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Plus, let's not forget that opals aren't diamonds. Diamonds fit into a pretty narrow range of colors and special properties. Opals are, even more than most colored stones, one of a kind pieces. Trying to grade them in this way seems a bit misguided to me, ultimately along the lines of labelling your stones "AAA Grade."

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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 12, 2016 5:31 pm 
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Stephen, the chart in question (I use a similar one) is specifically used to distinguish Australian opal body colors, white to black (not play of color) as defined in the new opal nomenclature adopted by the Australian Gemstone Industry Council. The process of actually quality-grading opals is horrendously more complicated than just that.

The body color chart has 9 sections, N9 being white and N1 pure black, with 7 gradations in between: Very Light, Light, two Dark choices, followed by 4 levels of Black saturation (N4 through N1).

Once the body color is determined, the real fun begins as many other factors come into play. At that point the old saying becomes true: diamond color-grading is easiest, 1 on a scale of 10, while opal is far more difficult, sitting in 10th position. The situation is further aggravated by the fact that no universal color-grading system for opal from all sources exists. Not all opals fit into the AGIC scheme.

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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 18, 2017 7:52 pm 
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So one letter/grade for color of body and one number/grade for color? What about like clear jelly opals?
Thanks!


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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 18, 2017 10:59 pm 
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This is a good article by GIA on opal grading
https://www.gia.edu/opal-quality-factor


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 Post subject: Re: Opal color grading
PostPosted: Fri Aug 18, 2017 11:09 pm 
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And this is the link to Anthony Smallwood's article on the method now used pretty widely in Australia and elsewhere:

http://www.opalsinformation.com/index.p ... s&dir=DESC

Scroll down to "A New Era for Opal Nomenclature" and open the link.

I just barely skimmed the GIA article though I'll study it later. I don't at this point know how it relates to the system Smallwood describes.

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