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 Post subject: It's not as stable as Australian opal, that's for certain
PostPosted: Thu Mar 26, 2009 1:01 pm 
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But it's not that bad. I've cut quite a bit, about half a kilo, of Ethiopian, ranging from the brown to the newer crystal, white, and other color finds.

Here's my take on it:

1) even the newer "won't crack" finds will crack. However, these cracks form around internal inclusions, so it varies from piece to piece.

2) the newer finds are not "sedimentary opal" even though the surrounding matrix looks like sand or clay. It's not. Looking at it under a microscope, it looks like a very fragile tuff with lots of phenocrysts in it to me. I've extracted a number of nicely formed feldspar crystal from the matrix, usually very transparent and small, which show no signs of weathering. So, you have an opal forming in a tuff flow, and one which is very felsic, so you tend not to have the dark colors seen in the rhyolite nodule Ethiopian opal, but I don't think, by itself, this has much bearing on stability.

3) The primary difference between Australian opal and Ethiopian opal is that Ethiopian opal is opal-CT, not opal-A. Gem quality opal-CT is made up of "leptospheres" rather than small amorphous silica spheres. The leptospheres are aggregates of cristobalite and tridymite platelets, and have a rather spiky or fibrous quality to them. I believe the cristobalite and tridymite material is originally volcanic in origin, since those two phases of quartz occur at higher temperature, and then it is dissolved via hydrothermal action, but not dissolved all the way down to single metasilicic acid units. What goes into solution are nanocrystals of cristobalite and tridymite. Once in solution, they react, probably due to changes in pH or temperature, to form suspended leptospheres, which then settle out and are cemented by the aggregated nanocrystal of cristobalite and tridymite. This probably accounts for the lower density and higher rate of hydrophane opal in Ethiopian opal when compared to Australian, since the aggregated CT leptospheres and nanocrystals don't pack as smoothly or cement as smoothly as amorphous silica. This is quite similar to most Brazilian and Mexican opal, actually, since those are often opal-CT as well.

So, about cutting it:

1) Work slow and keep cool. The opal-CT structure is not as rugged as opal-A, but can be cut successfully without further cracking or crazing.

2) Let the rough finish "natural" cracking before you work it. This is caused by internal stress once removed from the host material, and differences in expansion/contraction between remaining tuff filled vugs and the surrounding opal. This means you may buy nice blocky 20 carat or so rough, and end up cutting a lot of 1-2 carat stones, but it's much preferable to know what will cut without cracking and cut a small stable stone than it is to cut a large stone and then have it crack due to residual stress when you're almost done. I have yielded some nice ~10 carat pieces, but most come out below 4 carats.

3) prepare it by soaking it well, and removing all external matrix, with special attention paid to deep, narrow pockets in the surface. The material may already be stressed and will crack anyway, but you can reduce that tendency by removing all externally reachable matrix quickly, letting the material soak, and then letting it fully dry. Cracks will form. Let them. If the material doesn't split by itself, apply a little pressure with your thumbs and split it along the forming crack. You might want to wear gloves as freshly fractured opal can be very sharp. I've ended up with bloody gem studded thumbs a few times doing this.

4) Go ahead, cry a little. You'll probably want to. That giant nugget of blindingly bright rough is now 2-3 smallish and very jagged little nuggets which are so oddly shaped that they will require that much of the brilliant color be ground away to get an acceptably shaped stone. Ethiopian opal is a cruel mistress and it's OK to cry. Bargain for discounts on rough accordingly.

5) Keep hard wheel use to a minimum. Vibration and chatter, plus pressure against hard wheels, can cause further cracking. Cold dop with plumber's Goop as it sets up hard, but not too hard, and gives a nice cushion to the stone reducing chatter and vibration, as well as modulating the pressure you apply. Use a fairly aggressive wheel to cut out any visible internal inclusions ASAP, otherwise the expansion and contraction of repeated wetting will induce cracks to form around these. Again, it's OK to cry. Your jagged little nuggets are now 1.5 carat rubs, and a heartbreakingly large amount of stunning color has gone down the drain.

6) Sand up to about 1200 grit. The only good part of this process is that you'll most likely end up with nice medium to high dome stones that are solid color skin to skin.

7) Dry finish using a phenolic spool polisher starting at about 8000 diamond grit on up to about 50K grit. Keep the stone moving, and monitor for any heat buildup. For a final polish use a a paste made from pure carnauba wax and 50K diamond powder, apply it with a small rotary brush on a flexshaft or Dremel tool, use a tiny amount, high speed, low pressure, work it until the wax begins to flow, pause, and chase the wax all over the stone as it melts and flows. Let it cool fully, and give it one last going over with even overlapping strokes. Wipe the stone clean while still warm, and you're done.

A few things to watch out for:

1) crystal hydrophane can absorb the cold dopping compounds and form a stain. Remove these, carefully, with acetone once the stone is done.

2) don't try to "chase" a crack and remove it by cutting in the direction of its travel. Observe the shape of it, see it as a conchoidal fracture, and remove by cutting in perpendicular to the face of the fracture surface.

Once I figure out what I did with my camera, I'll post some pictures so you can see the result. I have stones that have been cut this way, with no stabilization treatment, that are just fine, no further cracking or crazing, after well over eight months. I'm waiting for them to reach a year before I sell them, just in case, but so far it appears that if you follow this method, you can cut a nice, stable, Ethiopian opal.


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 Post subject:
PostPosted: Thu Mar 26, 2009 7:39 pm 
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Great post JBert. Thanks!

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 Post subject: Photos
PostPosted: Fri Mar 27, 2009 12:33 am 
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As promised, photos. Unfortunately, I'm a terrible photographer.

Image


Here is a set of opals I cut from some of the "Gondar find" rough, about 30 carats total weight. The brightness really doesn't come through in this photo. nor does the remarkable three dimensionality of the patterns. The best thing about using a spool polisher with this material is that it's so bright that you get a little "laser light show" on the spool while polishing it. As you turn the stone, the moving face of the spool is lit up with rotating pools of color so bright and pure that it looks like you're shining an LED or a laser pointer on it. Most of these stones have clear flashes of color easily visible from ten feet away under typical indoor at night residential illumination. As with many super bright opals they appear brightest under dim light. They easily outshine $3000/carat black opals, with no need for a dark base to give contrast.

The patterns are also very interesting, usually sharp edged with well-defined regions of color. The relatively dull green one top and center has an astonishing hexagonal flagstone pattern that has to be seen in person to appreciate. The color is divided into very shard edged slightly irregular hexagons of blue and green. The red one, top left, has remarkable little hexagons of pure red, distributed three dimensionally through it. The camera I'm using just can't focus on the pattern.

One of these afternoons, when I have nothing else to do, I'll take a few of my small chips down to the SEM lab, thin section, HF etch, and SEM them, and post some pictures of the structure. I expect it will be very interesting.


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 Post subject:
PostPosted: Fri Mar 27, 2009 10:22 am 
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This came out today in the GIA Insider:

http://app.e2ma.net/campaign/7d4bddde0c ... e#article2

I guess we'll see more in the upcoming issue of G&G.

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 Post subject:
PostPosted: Sun Apr 19, 2009 12:25 am 
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The nodule-type brown Ethiopian opal is, for the most part, unstable and not predictably suitable for cabs, although it can be cut into attractive slices, and, once in a great while, you may get lucky and get a solid cab that does not crack. It certainly has value, but primarily as glorified specimens with terrific color and patterns. Almost everyone I've talked to about this opal recommends Opticon or similar treatments to stabilize it, but I consider anything treated like this as glorified specimens, no matter what the dealer tries to sell you on.

The crystal-like "desert" material, on the other hand, is terrific material that will soon have a huge impact on the opal world. It is sedimentary, not volcanic, in nature, and it is VERY stable, and can be cut like traditional Australian material - dopped with wax, shape/sand on diamond wheels, polished on felt with Cerium Oxide - with no ill effects. It is, at least currently, a fraction of the cost of comparable Australian crystal. Keep your eyes open, because you are gong to see a lot more of this awesome material coming on the market.


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 Post subject:
PostPosted: Sun Apr 19, 2009 10:47 pm 
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Larry Opal wrote:
The nodule-type brown Ethiopian opal is, for the most part, unstable and not predictably suitable for cabs, although it can be cut into attractive slices, and, once in a great while, you may get lucky and get a solid cab that does not crack. It certainly has value, but primarily as glorified specimens with terrific color and patterns. Almost everyone I've talked to about this opal recommends Opticon or similar treatments to stabilize it, but I consider anything treated like this as glorified specimens, no matter what the dealer tries to sell you on.

The crystal-like "desert" material, on the other hand, is terrific material that will soon have a huge impact on the opal world. It is sedimentary, not volcanic, in nature, and it is VERY stable, and can be cut like traditional Australian material - dopped with wax, shape/sand on diamond wheels, polished on felt with Cerium Oxide - with no ill effects. It is, at least currently, a fraction of the cost of comparable Australian crystal. Keep your eyes open, because you are going to see a lot more of this awesome material coming on the market.


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 Post subject:
PostPosted: Thu Apr 23, 2009 4:06 am 
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my 17ct faceted round ethiopian opal started cracking a few months ago (looks to have stopped) but only near the table, and I think it's now at the surface but it's hard to tell. It was cut 3-4 years at least before the cracks started showing :( but all the beautiful colour play is still there. Is there anyone who knows if I can do anything or send it to anyone who can opticon it or use another sealant to prevent it from shattering?


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 Post subject:
PostPosted: Wed Aug 19, 2009 5:11 pm 
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here is one i have rubbed and quickly run over a 1200 wheel.
the linearities you see in the photos are scratches.

http://www.mochakenya.com/opal/gonder/

this one is going in and out of water several times a day to see if the drying and hydration cyles will cause it to craze.

this stone is completely clean and sound at this point.
hehe.

I will update the page when it cracks with photos.


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 Post subject:
PostPosted: Wed Aug 19, 2009 7:27 pm 
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WOW!

I have no other words :shock:

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 Post subject:
PostPosted: Thu Aug 27, 2009 9:35 pm 
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i have updated the page with some new material and some correspondence from Mike Kelly

http://www.mochakenya.com/opal/gonder/


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 Post subject:
PostPosted: Tue Nov 03, 2009 5:41 pm 
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hello all,
FYI
to date NONE of the stones I have cut have cracked or crazed.
My associate has now cut over 500 stones from this material since starting in march 09 and NONE have cracked or crazed.

I am not getting a huge volume of this material, so it is all sold by the time i get it.

I have a waiting list of people wanting to buy rough.

One of the largest dealers in Europe is getting up to USD 500/ct for the cut stones.


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 Post subject:
PostPosted: Fri Nov 06, 2009 12:29 pm 
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I bot some samples in Tucson in Jan. Cut the stones. 2 came out some what milky. Not worth much. One came out reasonalbbly crystaliized and one well crystalized. All had red flashes. I'm holding till Tucson to see if there is any crazing . Until now, no.
White opal prices generally run from about $3 for miilky with weak green/blue flashes to about $$125 for gem white. Black opals are more.

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