May 31 through June 2—COLORADO SPRINGS, COLORADO: Annual show; Colorado Springs Mineralogical Society; Norris Penrose Event Center, Fri. 12-7, Sat. 10-5, Sun. 10-4
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Wed May 22, 2019 9:47 pm

All times are UTC - 4 hours




Post new topic Reply to topic  [ 10 posts ] 
Author Message
 Post subject: interesting opal
PostPosted: Wed Jul 19, 2006 12:44 am 
Offline
Valued Contributor
User avatar

Joined: Mon Mar 06, 2006 2:13 am
Posts: 103
Location: vienna, austria
hi all,

when sorting through our rare-stone-box i found this opal, which i bought from a brazilian dealer some 10 years ago and had completely forgotten about.

Imagethis really is an opal, photo shows colour in reflected (white) and transmitted (orange) light


the dealer told me it came from the state of minas gerais. we also have some yellow facetted opals that actually look more like yellow sapphire (except for the lustre) from brazil but i have never seen anything like this partly transparent and partly milky white stone before or after.

would anybody know more about the location?

_________________
Wiener Edelstein Zentrum (Vienna Gem Center)
http://www.edelsteine.at/recent-aquisitions/newe/
(re-sellers please register to see wholesale prices)
office@edelsteine.at


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jul 19, 2006 6:26 pm 
Offline
Platinum Member

Joined: Tue Nov 29, 2005 2:02 pm
Posts: 2647
Hi gemtrader,

Don't know about the location, but, that certainly is an unusual specimen, worthy of a collectors tray. Never seen any thing like it in opal.

Even the cut is unusual for opal.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jul 19, 2006 6:41 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 19595
Location: San Francisco
The only references I've seen to opals being mined in Brazil have been found in Piaui State.

They are sometimes treated with Opticon because of porosity.


Last edited by Barbra Voltaire on Wed Jul 19, 2006 9:19 pm, edited 1 time in total.

Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jul 19, 2006 7:08 pm 
Offline
Platinum Member

Joined: Tue Nov 29, 2005 2:02 pm
Posts: 2647
Found this looking around.
http://www.segnitopals.net.au/main/opal_world_2.html#


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Jul 19, 2006 9:16 pm 
Offline
Platinum Member
User avatar

Joined: Fri Feb 24, 2006 1:20 am
Posts: 2737
Location: Southern California, U.S.A.
Barbra and JB,

[img][img]http://img135.imageshack.us/img135/7341/brazilianopalcopyvr6.th.jpg[/img][/img]

Here's an example of Piaui boulder opals I photographed in a museum on a recent visit to Idar-Oberstein.

Brazil's Piaui State produces some of the finest precious opal in the world. The country also produces considerable amounts of yellow-to-red fire opal similar to the Mexican material mined in at least a couple of locations outside Piaui. The cloudiness in the posted stone above looks very much like a condition I've seen in numerous volcanic-formation opals from the Magdalena and Aguas Caliente regions of Mexico as well as other opal sites. I have a theory about what causes it but won't waste that much bandwidth here right now -- too much else to do.

Some quantity of really fine Piaui precious opal came onto the American market in the mid-1970s. I knew two of the importers and I cut quite a bit of it, mostly really fine crystal. I fell head-over-heels in love with it: it's extremely bright, very stable and considerably harder than Australian precious opal. Most of that production came from Pedro Segundo. Then one day it was no longer available. I heard stories about mining difficulties, problems with the government, partnership treachery, etc., mostly true I think. But recently I learned that the production since then has gone to Australia for resale as Australian opal which is better known and brings higher prices.

In the last several years the Brazilian opal industry seems to be stirring again with the opening of Boi Morto under the auspices of an American opal dealer. I saw some really fine rough stones a couple of years ago but most of what's trickling onto the American market now is already cut. Nearly all is either crystal or very fine white potch base with nearly solid multicolor flashfire. I've seen no black or dark opals comparable to those from Lightning Ridge, Oz, but who cares? This is high-end opal!

I think I know where to locate some images and will try to post one or two later.


Top
 Profile  
Reply with quote  
 Post subject: Brazilian Opal
PostPosted: Wed Jul 19, 2006 9:37 pm 
Offline
Platinum Member
User avatar

Joined: Fri Feb 24, 2006 1:20 am
Posts: 2737
Location: Southern California, U.S.A.
Image

Here's one: a 6.55 ct. stone measuring 19 x 12 x5 mm. that's priced so well I'm very tempted....

ROM


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Thu Jul 20, 2006 1:45 am 
Offline
Platinum Member
User avatar

Joined: Fri Feb 24, 2006 1:20 am
Posts: 2737
Location: Southern California, U.S.A.
Image

Back to the question about the cloudy opal at the top of the thread. The blue common opal in this image came from inside a special kind of Oregon thunderegg. I purchased this piece among others in Tucson about 2 1/2 years ago as carving material.

Originally the entire piece was about the color of the sky on a fine
September day in the northern hemisphere. It was very translucent and simply gorgeous! Because I've had some experience with this material I set it aside to see how time would treat it. :cry: Not well, as can be seen.

I can't explain the exact process technically but I believe the increasing cloudiness is one of the things that can happen when opal of certain kinds with too much water content is suddenly exposed to air. I collected similar opal nodules in my younger days and when opened they were often wet inside. The beautiful nearly transparent blue opal inside metamorphosed into opaque whitish material that really didn't resemble opal within a couple of years.

This 'mountain opal' as it's sometimes called is directly related to volcanic activity. It forms inside thundereggs or is deposited in masses as it precipitates out of hydrothermal solutions. The precious opal from Australia's Great Artesian Basin is sedimentary in nature and is considered much more stable. Why no one really seems to know. :smt017

I'm not a mineralogist but based on my own observations I think opal of the type illustrated actually undergoes a chemical change of some sort as water escapes and other reactions take place. It seems to revert to cristobalite or something close to it. Anyhow, that's my conclusion from observations over a long period. I'm very open to other ideas.


Top
 Profile  
Reply with quote  
 Post subject: opal
PostPosted: Thu Jul 20, 2006 2:02 am 
Offline
Valued Contributor
User avatar

Joined: Mon Mar 06, 2006 2:13 am
Posts: 103
Location: vienna, austria
thanks, rom,

you are a true fountain of information.

i want to add that the bicolored opal shown in my original post seems to be stable. it has not changed a bit during ten years, no cracks or dull spots developed and the milky part stayed the same size.

_________________
Wiener Edelstein Zentrum (Vienna Gem Center)
http://www.edelsteine.at/recent-aquisitions/newe/
(re-sellers please register to see wholesale prices)
office@edelsteine.at


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Thu Jul 20, 2006 4:33 pm 
Offline
Platinum Member
User avatar

Joined: Wed Apr 26, 2006 12:44 am
Posts: 2045
Location: San Francisco
Rom, your water content on the opal was too high, so it evaporated once exposed to low humidity. I have 5 jars of beautiful mexican opal that now looks like tile grout! Thought at least I could tile them into the shower wall and they'd still be pretty years later. .which is about all they are good for..

How much they asking for that black 6.55 ct. Make sure you have a return on it, it has a bit of a grey mist on the surface and they ALWAYS photograph 3-4 grades better than they look in reality. It sure LOOKS nice.. ;)

Thats a nice clear stone there gemtrader. I've never seen anything like it, even in the translucent opals. It does make me wonder though, how the opalization process could have a partial fill like that. Since it is water and silicon dioxide spheres make the colors, and the size of the spheres determines the wavelength of light reflected ( big spheres = red, small spheres = blue/green). What kind of interface does it have between the opalescent layer and the clear layer. Have you put this under a microscope? At 200-400x you should see some regular sphere structure. The packing is the giveaway..

Anyway, it just looks very strange to me, my first impression when I saw it was ".. Oh, that's an interesting type of coating on quartz..."

Would be kind of interesting to find out more about it.


Top
 Profile  
Reply with quote  
 Post subject: microscope
PostPosted: Fri Jul 21, 2006 1:51 am 
Offline
Valued Contributor
User avatar

Joined: Mon Mar 06, 2006 2:13 am
Posts: 103
Location: vienna, austria
jleb, i´ve run it through the lab before i bought it because id just did not look like opal to me - and still doesn´t. it´s opal allright.
we checked R.I and density and the orange opalescence in transmitted light is conclusive.
i don´t remember if we microscoped it. but even if we did the magnification was 120x max. we had a microscope going up to 400x times but i tried to avoid working on it unless absolutely inevitable. finding an inclusion, moving the focus away from it and then trying to find and re-focus it again was a nightmare...
unfortunately that lab does not exist any more.
i will, however, give it to the austrian gemstone institute of the vienna museum of natural sciences. they´d be interested - and might even buy it :wink:

by the way: the vienna museum houses the third largest gem collection of the world, after the smithsonian and the london museum of natural sciences. if you ever come to vienna, don´t miss it...

_________________
Wiener Edelstein Zentrum (Vienna Gem Center)
http://www.edelsteine.at/recent-aquisitions/newe/
(re-sellers please register to see wholesale prices)
office@edelsteine.at


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 10 posts ] 

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 0 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock