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 Post subject: Question about honeycomb opal
PostPosted: Sat Dec 29, 2012 3:44 pm 
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I have an opportunity to purchase a fairly large honeycomb opal from a private individual. Since I know very little about them, I did a little research and it seems that everything I find lists the source of these as Ethiopian. However, the seller says it is NOT Ethiopian. It's a fairly large stone, approx. 23mm x 16mm x 9mm in size, and has mostly yellow, green and orange colors. My question is this: what are the other sources of honeycomb opal since everything I can find says Ethiopia, yet the seller says it is not Ethiopian? And are the opals from other sources as valuable as the ones from Ethiopia?
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 Post subject: Re: Question about honeycomb opal
PostPosted: Mon Dec 31, 2012 10:15 am 
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I know that Australia produces some honeycomb opals, but I believe that they are very uncommon. The most common honeycomb opals are crystal or white base from Welo, Ethiopia, but I've seen lots of brown and black base from Shewa, Mezezo, and Welo, all regions of Ethiopia. You can ask the dealer if it's hydrophane (absorbs water). If so, it's most likely Welo. Do you have any pictures?

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 Post subject: Re: Question about honeycomb opal
PostPosted: Wed Jan 02, 2013 5:48 pm 
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Thank you for the reply. There is no brown or black base on this stone. This is the only picture I have. It was taken with my phone in order to show my wife, so it isn't very good.


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 Post subject: Re: Question about honeycomb opal
PostPosted: Wed Jan 02, 2013 5:55 pm 
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Sorry for the redundancy! The owner emailed me a picture he had of the piece, so here is a much better picture of it.

Thanks for your help!


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 Post subject: Re: Question about honeycomb opal
PostPosted: Wed Jan 02, 2013 10:32 pm 
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ktriddle wrote:
My question is this: what are the other sources of honeycomb opal since everything I can find says Ethiopia, yet the seller says it is not Ethiopian? And are the opals from other sources as valuable as the ones from Ethiopia?
Thanks!

Hi, that's a beautiful opal! I've specialized in cutting opal from worldwide sources for more years than I care to mention. I've seen lots of opal from many places but have never encountered such well-defined "honeycomb" structure from any source but Ethiopia.

A "Gems & Gemology" article (Summer 2010) on Wello (Ethiopian) opal notes that what the authors call "digit patterns" (the "honeycomb" appearance common in Ethiopian opal from both Shewa and Wegel Tena) had been observed only once in Australian opal by one of the authors, along with one other example from an unspecified source. I've cut lots of Australian opal and have never seen patterns anything like those in your images.

If the opal is hydrophane (sticks slightly to the tip of your tongue or to a wet fingertip) my guess is it's Ethiopian (Wello). If it doesn't we have an interesting mystery. I'd think it's up to the seller to provide information about the source in any event.

As to your question about values, in general Ethiopian opal sells for a discount compared to opals from other established sources. There are many reasons for this, especially supply/demand: there's suddenly a lot of it on the market and trade channels from Ethiopia are not well developed. Probably the world's most valuable opals are the fine black stones from Lightning Ridge, Australia. Very fine opals from other Australian fields, as well as some from Brazil, also command very high prices.

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 Post subject: Re: Question about honeycomb opal
PostPosted: Wed Jan 02, 2013 11:18 pm 
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Somewhere amid my clutter I have a small piece - less than the size of your little fingernail - of Mexican opal with a muted but definitely perceptible honeycomb pattern in a semi-translucent red/orange base color. I saw it not long ago. If I find it again I'll take a picture.

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 Post subject: Re: Question about honeycomb opal
PostPosted: Thu Jan 03, 2013 1:47 am 
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Hans, just as idle speculation I wonder if that structure is somehow related to whatever process it is that creates hydrophane? Mexican and Indonesian opal seem to me most similar to Ethiopian of all the opals I've worked with: they're volcanic in origin and some are hydrophanes -- but with a difference!

Years ago I bought a parcel of Mexican opal rough. I remember the wonder and amazement I felt when a piece I'd decided to cut into a cab suddenly became alive with bright full-spectrum play of color as it absorbed coolant water from my grinding wheels. I also recall the disappointment I felt when it dried and reverted to a mundane hunk of orange-ish potch.

Wello opal is "reverse" hydrophane. It loses color when immersed in water, the exact opposite of my Mexican stone. Go figure!

I know some precious opal makes a formational structural transition from amorphous to semi-crystalline. I wonder if the "digital aka honeycomb" appearance is related to the change from amorphous to crystalline and represents proto-crystallization? (I have no idea how any of this might relate to hydrophaneity).

I think it's time to bring Marco Campos-Venuti into the conversation. He seems to have a theory that might explain all this. I'll flag him to this thread and hope he can shed some light on the process.

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 Post subject: Re: Question about honeycomb opal
PostPosted: Thu Jan 03, 2013 10:19 am 
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I'm with Rick, that looks like a very nice example of the honeycomb material from Wello, Ethiopia.

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 Post subject: Re: Question about honeycomb opal
PostPosted: Thu Jan 03, 2013 11:28 am 
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Thank you all for the information you've provided. I've found out that the piece was originally purchased in Kabul, Afghanistan, and the buyer was told only that it was a honeycomb Opal, but that it was not an Ethiopian Opal. So, I guess I'm back to square one. It's a pretty piece, and my wife likes it, so I guess it doesn't really matter where it's from if I can get it for a good price! It just piqued my curiosity about its origin.


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 Post subject: Re: Question about honeycomb opal
PostPosted: Thu Jan 03, 2013 2:15 pm 
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Knowing it was purchased in Afghanistan I'd now bet money it's Ethiopian. Lots of Ethiopian opal is smuggled out of the country. At the point of sale it's often represented as being non-Ethiopian to avoid possible legal problems. It's often sold as Somalian in origin.

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 Post subject: Re: Question about honeycomb opal
PostPosted: Thu Jan 03, 2013 10:53 pm 
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I think the explanation is probably pretty simple. The columnar structure is similar to columnar basalts that result from contraction from cooling parallel to the heat gradient (see: top of Devil's Postpile http://2.bp.blogspot.com/-87I-0o4Xi7k/TkLC0gt5bPI/AAAAAAAAAHg/PgLlYSM47b0/s1600/devils_postpile3.jpg. I would guess that it's something similar, but with drying: the opal forms wet, dries, contracts and cracks. Then there's a second episode of opal formation which fills the cracks. What would have made this particular mine produce a lot, and others to produce very little, I don't know--but the wello opal is supposed to have formed pretty quickly, since there is still carbon in the fossilized twigs contained (I read that somewhere, I'm sure).

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 Post subject: Re: Question about honeycomb opal
PostPosted: Wed Jan 16, 2013 5:06 pm 
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I got busy and forgot to follow up on this. A personal communication from Marco Campos Venuti concerning the origin of honeycomb structure in Ethiopian opal contained the following information:

"At the CIGES 2012, the Italian gemological congress, Benjamin Rondeau presented with a group of researchers a nice interpretation...the honeycomb structure is due to a dissolution surface that attacked around the color blotches."

He said the Rondeau dissertation would be published (in Italian) as an abstract with photographs in the next edition of his Rivista Gemmologica Italiana.

"The pictures are very clear," he wrote "but I cannot put it on-line. I'm sure that a full paper will be published soon. Maybe in Gems & Gemology.

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 Post subject: Re: Question about honeycomb opal
PostPosted: Thu Jan 17, 2013 4:59 am 
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i've been at the conference and, have to say, Rondeau's lecture was likely the most interesting one (and this comes from someone who's not particularly fond in opals.... :oops: ), like Marco i shoot many pictures but, for the same reason i wouldn't post them.........but this one:

Image

......i really can't wait for the paper's official release....... 8) 8) 8)

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alberto

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 Post subject: Re: Question about honeycomb opal
PostPosted: Thu Jan 17, 2013 5:33 pm 
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"Dissolution surface" - now that has got my rummaging through my mind's various compartments of odd and unclassified information trying to come up with an analogy. Perhaps like the dissolution at the grain boundaries of an etched metal? I'm very curious to hear the outcome.

Also somewhere among my equally unclassified "stuff" stored in tote bins here and there and elsewhere is a copy of "The Opal Synthesis Handbook" (author escapes me). It was a self published photocopy booklet in a a greenish cardstock cover that described a process for synthesizing opal from organic silica bearing reagents in a tall cylindical retort. I recall reading that as this stuff settled and compacted at the bottom of the retort it assumed a columnar structure (not unlike Gilson) but also have a dim memory that it was described as having a honeycomb pattern when seen from the top.

Cheers all,
Hans Durstling
Moncton, Canada

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 Post subject: Re: Question about honeycomb opal
PostPosted: Sat Jan 19, 2013 11:13 am 
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Hans Durstling wrote:
...rummaging through my mind's various compartments of odd and unclassified information...


That's why you're such a creative genius: you used to be a superspy! ;)

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