How to Cut, Facet & Polish Opal Cabochons - Moriartys Gem Art
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  • How to Cut, Facet & Polish Opal Cabochons

    by Jeff Moriarty April 14, 2023


    Video Transcription

    Hi, I'm Steve Moriarty from Moriarty's Gem Art. Today I'm going to show you my techniques for cabbing Welo opal, which comes from just north of Addis Ababa here in Ethiopia.

    Now, this is really a beautiful piece of Welo opal. It weighs 32 carats. As you can see, there's a lot of matrix on the outside, and sometimes this matrix goes deep into the stone in deep pockets. So I'm hoping that we don't have that issue so I can cab this stone today. Sometimes when it goes very deep either you lose a lot of material or you've got to sculpt it.

    So now we've got the matrix off the stone, and we've lost about 24%. We've gone from 32 carat to 24 carat. So the next step is to go to one of the coarser wheels, these diamond wheels. This is probably about a 180. I'll take the bulges off of it, and then look, see what kind of shape I can finally get out of it.

    I'm just taking the rough edges out, and try and get some of the dimples out of it and the rest of the matrix, and then I'll decide then what the shape could be. Got some deep holes here and here that create problems with getting any kind of yield out of it, comparatively.

    I've still got a couple of depressions here to get out. The shape's getting better.

    I've got the basic shape now. All the holes and divots are out of it. So now I'll move on to a 1,200 belt and do a little refining of the shape and smoothen this out.

    Okay, so even though this is a pretty soft material, instead of going to the 1,200 right away, I have a little more work to do on it, so I'm going to the 400 belt.

    I'm using very light pressure just to try and get some of the lines that are in this from the core slab.

    Now I'm on a 1,200 belt. I can use a little more pressure and even out the lines that are in it. I move quickly to get a nice, even shape. Then wipe it dry so I can see the surface of the stone and make sure I got all the imperfections out. It's looking pretty smooth. Got a little bit on the back yet.

    All right, finalize my shape.

    It's looking pretty good. Now, the last thing I do to help the setter is try and bevel this edge here. And now we can move on to 8,000. Now the 8,000, I even use a little more pressure [inaudible 00:08:11] water. Get that beveled edge again.

    So now is when you've got to get it completely smooth, because the final polish is not to take out lines. It's looking pretty good. I think we're ready to polish.

    All right, so I finished this on 8,000, and the finish is very good on it. Very few lines. The lines are very small. Now we're going to go to a final polish on leather. I use cerium oxide for the final finish. Fairly hard pressure, so it presses into the leather and gives you a nice, rounded surface. I'll wipe it dry.

    One safety issue. You should be wearing masks, but I can't talk and wear a mask during the process, because both cerium and the opal can be a little hazardous in the long term.

    Now I'm pretty satisfied with that. This is not showing its color because it's wet now. With all Ethiopian opals, when they get wet they lose their color and get very transparent. But I think this is going to be [inaudible 00:12:15] few minutes.

    Okay, this is our finished stone. It came out just a little over 12 carat, so we only got 37% yield out of it, a little lower than normal, but all this material has bad yield just because of those deep pockets that are in them. So in a couple days we'll see what this stone really looks like.

    Jeff Moriarty
    Jeff Moriarty

    Jeff Moriarty has been in the jewelry industry for almost 20 years. His family now only owns a retail jewelry store in Crown Point, Indiana, but he also travels the world with his father in search of rare gemstones.

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