CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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 Post subject: fire opal vs. carnelian
PostPosted: Fri Aug 23, 2019 8:05 am 
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Some years ago I bought a handful of faceted yellowish/orange/reddish translucent stones tagged as fire opals. The price was so low that I doubted the "opal" tag and decided to try to identify them.

My first thought was carnelian, as some of the stones under loupe showed mottled zones similar to those seen in chalcedony.

As the only major difference between opal and carnelian is the structure - amorphous vs. polycrystalline, I decided to check them with a polariscope.

All stones except one were bright in the polarsicope field, no matter the rotation of the base. For me that was consistent with polycrystalline structure, therefore I tagged those as carnelians. The single stone that remained dark (at any orientation) was assumed to be opal.

I know that one test is not sufficient for positive identification, but I am curious if my thinking is right or wrong, regarding the differentiation between fire opal and carnelian.

Thank you in advance for answering this post!


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 Post subject: Re: fire opal vs. carnelian
PostPosted: Fri Aug 23, 2019 9:00 am 
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Specific gravity would be a good addition to this. Typically carnelian will show agate banding (especially since most 'carnelian' on the mass market is dyed Brazilian agate, particularly faceted examples sols on JTV which I see now and then) so microscopic examination would be a good idea.

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 Post subject: Re: fire opal vs. carnelian
PostPosted: Mon Sep 02, 2019 9:28 am 
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The RI of fire opal is <1.45, the RI of carnelian is > 1.53

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