social media icons

Blog Archive

Monday, July 16, 2012

Loving Ethiopian Opals.

For the people who know me, they are also mostly likely to know that I often qualify Ethiopian opals as "the best opals ever"! Is that a biased opinion? Maybe... But for sure, some of them are just exceptional! 
Opal from Wollo, Ethiopia. One of my favorites, it has everything: the play-of-color, the perfection of the structure (pseudo-monocrystal), and the digit pattern. Photo and sample: F. Mazzero, extracted from Rondeau et al. (2011).

What is an opal?


Opal is an amorphous (opal-A) to poorly crystallized (opal-CT) silica that contains water (SiO2, nH20). The water content usually varies from 4 to 10%, but in some cases, it has been reported to be only 0.8% and as high as 21%. It has a specific gravity of about 2, but again, with extreme cases reported down to 0.67 for extremely porous opals (they would actually float!) up to 2.3. Its index of refraction is in between 1.37 and 1.47 with a main value at 1.44. Hardness varies between 5.5 and 6.5, it is therefore quite a soft gemstone (softer than quartz, which has a hardness of 7).
Some opals (either opal-A or -CT) can present what we call play-of-color, which is the fiery rainbow of colors that dances around when moving the stone. This effect is due to the diffraction of light on a network of spheres (for opal-A) or lephispheres (for opal-CT). To learn more about the play-of-color of opals, and why it is, most of the time, absent from opals (the opals are then referred as common opals), you can download here the article from Gaillou et al., (2008).
White, play-of-color opal-A from Tecopa, California. The silica spheres are arranged in a nicely organized network (not the staking faults though!). This opal is very porous, due to the lack of cement in between the spheres.
 White play-of-color opal-CT from Mexico (after HF etching). The network is made out of silica "lephispheres", giving also rise to the play-of-color.


Click to read more about what makes Ethiopian opals special!



What so special about Ethiopian opals?

In terms of play-of-color, that is the locality where you will find the most diversity.
From this opal, with patches of colors representing the entire rainbow:
Opal from Wollo, Ethiopia. Copyright: Opalinda.

To this opal with an homogeneous play-of-color:
Opal from Wollo, Ethiopia. Photo by Mazzero, extracted from Rondeau et al., 2011.

You can even have some layered ones:
Opal from Wollo, Ethiopia. Photo by Robert Weldon, extracted from Rondeau et al., 2010.


Now, there are also some freaks out there: the perfect pseudo-crystals. While most opals have their silica spheres ordered at a small scales (giving the patches of play-of-color), some have a long distance order, achieving up to one centimeter or more. So there is no patches of play-of-color, but points displaying spectral colors.
This very unusual opal from Wollo (4.43 ct) displays a perfect diffraction pattern. Spectral colors formed small points that moved in a synchronized fashion and changed color when the light source was moved. Photo by B. Rondeau, extracted from Rondeau et al. (2010).

There are also some "rainbow" opals, where each patches of play-of-color display the entire rainbow of spectral colors. As B. Rondeau mentioned on this website, "The exact reason for these rainbows to appear remains unknown, but it could result from the diffraction on a curved network or on a striated surface". Click here to see a video of this effect.
Patches of pure spectral color forming a rainbow. Photos by B. Rondeau. Extracted from: above: Rondeau et al., 2011; below Rondeau et al., 2010 (9.51 ct stone).

In opals from both Mezezo and Wollo, "digit patterns" are sometimes visible, giving them this (almost) unique character. So far, I've seen this pattern only in some rare opals from Nevada (never as strongly defined though). It consists of a columnar structure with the columns with transparent opal presenting a play-of-color, while the walls of the columns are made of a milky or opaque, common opal.
Beautiful digit pattern exhibited by this opal from Mezezo, Ethiopia. Notice that it also behaves like a single crystal for the play-of-color. Photo: F. Mazzero.
video
This is the same opal as in the first picture of this post. Just stunning. The columnar is seen from above, so only cells can be seen. The inside of the cells are made of transparent play-of-color opal, while the thin walls are made of milky common opal. Video and picture taken at the Gem & Mineral show in Tucson, Opalinda & Eyaopal booth.

Icing on the cake: some opals from Wollo contain fossilized plants (stems, twigs and roots)! You can have the typical silicification, with silica filling voids of existing plants. This silicification phenomenon is diagenetic process that occurs after sediment burial.
Wollo opal that filled wood. Photo: Mazzero, extracted from Rondeau et al., 2011.

But most commonly, plant fossils are embedded in Wollo opal, suggesting that the opal formed in a silica-rich water where plants developed, that is, in a soil at the surface of the parent rock.
A plant fossil in Wollo opal (main stem about 1mm in width). Photo: F. Mazzero. From: Rondeau et al., 2012.
A stem (about 1mm wide) in Wollo opal. Photo: B. Rondeau. From: Rondeau et al., 2012.
More stems in Wollo opal. Photo: B. Rondeau. From: Rondeau et al., 2010.


Are Ethiopian opals stable?

The answer is: just as much as Australian opal. Meaning: There is always a risk that an opal with crack ("craze") or turn milky / white. A brilliant article has been recently published on the subject by Rondeau et al., 2011, in InColor. So, as for Australian opals, the best is to buy an Ethiopian opal from a trustworthy dealer. Most of the time, an opal begins to craze or become white either after being unearthed, or after cutting. It is better to keep an opal in storage for a while before selling it. I know that some Australian dealers keep their opals for 2 years in storage before selling them... And that is wise. All in all, unstable opals have been reported from worldwide localities, unfortunately.
Here is an example of a crazed Australian opal:

"Crazed" (cracked) Australian opal, viewed in transmitted light (left) and reflective light (right). Photo by F. Mazzero, from Rondeau et al., 2011.

And an example of an Ethiopian one.
"Crazed" (cracked) Ethiopian opal. Photo by F. Mazzero, from Rondeau et al., 2011.

Ethiopian opals suffer from the fact that they are hydrophane ("porous" opals, gaining weight when put in water). But as reported in Rondeau et al., 2011, there is no obvious link between unstable and hydrophane opals. However, so far, a much higher percentage of opals from Wollo (Tenel Wega, new deposit) has been recognized stable compared to opals from Mezezo (first found in Ethiopia).

Wollo (Wegel Tena, Ethiopia) opal: a resistant opal!

Here is a quote from Rondeau et al., 2010, which says it all:
"We noticed by accident that Wegel Tena (Wollo) opals could sustain a fall from 1.5 m onto a concrete floor with no visible damage, even under the microscope. Repetition of this test on five oval cabochons did not produce any sign of damage. The same experiment with five oval cabochons from the Mezezo deposit and three oval cabochons of white opal from Australia (including one boulder opal) led to breakage of all samples."

And I've seen it live. Dealer F. Mazzero doesn't hesitate to throw his beads of Ethiopian opals on his glass booth at Tucson! It's worth a stop (on top of all the high quality opals he has on display)!

Also... Just to let you know: Dior and Van Cleef and Arpels now use a lot of Ethiopian opals for their creations...
Paysage d'opale (California rêverie collection, Van Cleef and Arpels): A 100.11ct opal from Ethiopia. Copyright: Van Cleef and Arpels.

What's next?
Getting a donation of Ethiopian opals for the Gem Vault!


12 comments:

  1. Don't hesitate to leave comments or questions in this section!

    ReplyDelete
    Replies
    1. Elo,

      faudrait revoir les liens sur les videos du site GemNantes. Ils n'ont plus l'air de fonctionner vu d'ici. Depuis que c'est relooké, la strucutre a changé, donc lesl iens aussi....
      Bises

      manu

      Delete
  2. Bonjour, concernant la première photo d'opale pseudo mono cristal, l'aspect peau de serpent. Qu'est ce qui provoque cela ? Est ce l'arrangement des sphères qui sont prises dans des macles très réguliers ou un autre phénomène de formation de structures internes. Sont elles hydrophanes (collent à la langue) et est ce que ce phénomène se voit sur les bruts ? J. FEREIRE

    ReplyDelete
  3. Bonjour,

    Malheureusement, à ce jour, nous ne savons pas pourquoi ces opales peuvent présenter cet effet peau de serpent (digit pattern). Mais cet effet semble très spécifique aux conditions reignant en Ethiopie (ainsi qu'au Nevada, qui possèdent également des opales dans un environnement sédimentaire avec formation dans un sol). Celà n'a absolument rien à voir avec l'arrangement des sphères. Plutôt aux conditions de sédimentation. Plus de recherche à faire sur ce sujet, mais ça fait quelques années que nous nous posons la question !
    Et oui, cet effet se voit sur les bruts. Consultez le site d'Opalinda pour voir quelques exemples : http://www.opalinda.com/

    La plupart des opales Ethiopiennes sont hydrophanes (mais quand elles sont polies, elles ne collent plus à la langue ! test à faire sur du brut). Mais celà n'enlève rien de leur splendeur. Au contraire: pour celles qui sont un peu opalescente (laiteuse), les immerger dans l'eau les rend encore plus transparentes, accentuant (ou tout simplement faisant apparaître) le jeu de couleur. Certaines opales n'aiment pas être mises dans l'eau puis séchées à l'air libre, et finissent par craquer si le processus est répété. Mais encore une fois, si vous achetez chez un bon dealer, vous ne verrez pas ce genre de problème (les opales doivent être testées avant d'être mises sur le marché... quand on achète du brut, c'est risqué, et il faut le savoir).
    Le fait que ces opales soient hydrophanes les rendent aptent au traitement (voir l'article dans G&G: http://www.gia.edu/research-resources/gems-gemology/issues/winter2011-contents/winter-2011-renfro.html)

    Merci pour ce commentaire ! En espérant avoir répondu à vos questions.

    ReplyDelete
  4. bonjour
    merci pour ces reponces just pour infos je taille des pierres depuis 8 ans et je possede quelques 200ecchantillons de diverses opales communes comme nobles de divers pays.je fais partie de l'afg de lyon et tout comme JP GAUTHIER je souffre de l' opalinite aigue . C'est lui qui m'a aider a comprendre certains passage de votre these sur l' opale ainsi que les 2 dug sur l'opale ( b.ronreau et celui sur l'opale d'ethiopie).Depuis peut je pré prépare les lammelles d'opales de +ou- 2 dixième collées sur verre pour certaines analyses pour b rondeau que j'ai rencontré a sainte marie l année dernière.Je suis au crg de nantes.Autre facteur "agravant"je suis pationné par tout ce qui est éffet (chatoyance asterisme ect..).Je possede quelques echantillons assez rares (pollucite apophillite etc..a differents effets).Donc si vous avez des questions sur certaines pierres n'hesitez pas..Q uestions est ce que les opales rouges irrisées peuvent etre appelées opales de feux. Cela a fait debat dernierement a l'afg est ce le fer qui en est l 'origine meme si il est trop diffus pour le percevoir aux instrument de labo.
    merci encore
    jacques



    comme

    ReplyDelete
  5. Bonjour Jacques,

    Bon, vous connaissez donc mon passé d'opalénophile également ! Vous préparez des opales d'Ethiopie pour Benj ? Eh bien, bon courage ! Pas trop de polir ce genre de matériel sans faire de trou ou sans ne rien casser ?
    Il va falloir que vous m'envoyiez une photo de l'opale rouge dont vous me parlez. Elle vient d'où ?
    Bien à vous,
    Eloïse

    ReplyDelete
  6. bonjour
    l'opale rouge viendrait d'éthiopie vraissemblablement tirée d'un nodule car les zones plus claires avaient des traces de gangue.pour les photos je les enveraient en septembre avec d'autres car jp gauthier en possedent quelques unes qui sont interressantes et il est en corse pour le moment et je vais pas tarder a partir egalement en vacances aussi.
    donc en septembre pour plus d'infos et de photos
    amicalement
    jacques

    ReplyDelete
  7. I really enjoyed this article - more please!

    Jake
    Earthquest Minerals

    ReplyDelete
  8. Dear Eloise,

    A lovely article with some excellent images. Would it be possible to use some of your SEM images in teaching materials? They really are excellent illustrations of the opal A and opal-CT structures. Full accreditation would of course be given!

    Sincerely,

    Dr. Dan Morgan
    University of Leeds, UK.

    ReplyDelete
  9. The images can indeed be used as teaching materials, as long as you give us accreditation, indeed! I have plenty more available. I guess you checked out the "common opals" article already? http://mineralsciences.si.edu/staff/pages/gaillou/Gaillou-etal_2008_CommonOpalStruc.pdf

    ReplyDelete
  10. Hi Eloise;

    I'll try this one more time. Sadly I wasn't able to get through your computer access block the first time (being old fashioned has it's disadvantages:-))

    I've developed a process for salvaging precious opal from mine run opal rough that takes about 20 minutes per piece. I've also done some tests on 525 pieces of salvaged Welo (Wollo; Wolo) Ethiopian Opal with the following results:

    1. Welo Ethiopian Opal seems to be very resistant to stress (dropping, dipping into very hot and very cold water, not to mention heat from dry grinding and slicing and vibration from tumbling, grinding and slicing).
    2. 40 pieces out of 450 (about 9%) turned chalky after being soaked in 67-degree water. However, I’ve provided very cheap and simple ways to identify, separate, and document these pieces. Being able to assure potential owners that the play of color isn’t affected or will return in a defined length of time will have a positive effect on the desirability (value) of a piece.
    3. For the Welo Opal showing a play of color when they arrive as pieces of rough, every one maintained that play of color throughout their grinding, slicing, tumbling, and extended soaking. This was a very pleasant surprise.
    4. WARNING: Excessive and relatively fast temperature changes will cause fracturing. My results, when starting with dry pieces of opal having internal temperatures of about 76 degrees, are the following:
    a. Placing 5 pieces in 108-degree water for 5 seconds resulted in no fracturing.
    b. Placing 10 pieces in 112-degree water for 10 seconds resulted in 1 piece fracturing (10%).
    c. Soaking 15 fractured pieces in 78-degree water for 1 hour resulted in no additional fracturing.
    d. Soaking 450 pieces in 67-degree water for 1 hour resulted in 51 pieces fracturing (11%).
    e. Soaking 351 un-fractured pieces in 67-degree water for a second hour resulted in an additional 10 pieces fracturing (3%).
    f. Soaking 120 un-fractured pieces in 67-degree water for a third hour after waiting about two weeks resulted in no additional fracturing.
    g. WARNING: Heating 30 pieces of opal from 76 degrees to 105 degrees over a 1-hour period by transporting them in a car resulted in 16 pieces fracturing (53%).
    h. WARNING: Dropping 30 pieces of opal having a body temperature of 76 degrees into water with a temperature of less than 67 degrees and leaving the pieces in the water for an hour can cause at least 11 to fracture (37%).
    5. 10 different pieces of salvaged opal that I tossed on a concrete sidewalk showed absolutely no visible fracture lines, scrapes, or marks of any kind.
    6. 5 finished opal cabochons that I also tossed on a concrete sidewalk showed absolutely no visible fracture lines, scrapes, or marks of any kind.
    7. Based upon the present condition of the salvaged pieces of gem quality opal I’ve accumulated over the last 2½ years, the low humidity and steady (72- to 76-degree) temperatures I’ve subjected the pieces to seem to have little effect on the quality and stability of the opal.

    I would like to send you 17 pieces of salvaged precious opal nodules totaling 98.5 carats and 4 pieces of mine run opal rough totaling 28.0 so you can see and handle naturally shaped Welo Opal. There's no catches or tricks and you have my permission to do with them as you will.

    Sincerely,
    Bob Nolan
    North Las Vegas, Nevada

    ReplyDelete
    Replies
    1. Hi Bob,

      Could you leave me your email address so I can contact you personally?
      You can contact me at: egaillou at nmh.org

      Delete