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Author Topic: Question about cutting Spencer Idaho opal  (Read 1327 times)

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ileney

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Question about cutting Spencer Idaho opal
« on: March 27, 2017, 11:21:14 AM »

I just bought my first piece of Spencer opal rough. It weighs about 5 ounces, is about 3" x 2", and two thirds of it appears to be a tannish matrix, and the rest white opal with two fire lines, one of them looking fairly strong from the side, about 1mm thick, running almost the entire length of the stone, the other thin and looking somewhat unpromising. I tried taking a picture but you can't see the fire lines at all and it looks like a plain rock!

My question is 1) can this be treated the same as Coober Pedy and other white opal 2) is it going to crack like crazy (it looks translucent and cracky on top by the weaker fire line, but more opaque and stable directly under the better looking fire line which is further down) 3) is there likely to be fire in the tan matrix (I ask because it looks like there is some bright red fire in one spot on the matrix, but I suspect there may be a miniscule coating of white opal I can't see over the matrix causing this?)

Is there any advice or information you can offer specific to Spencer Opal as opposed to Coober Pedy and other white opals before I start cutting and grinding? Many thanks in advance for any help you can offer.
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gemfeller

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Re: Question about cutting Spencer Idaho opal
« Reply #1 on: March 27, 2017, 12:07:34 PM »

Spencer opal is cut almost exclusively as triplets.  I've cut many of them.  If you have a thick fire line it might be cut as a solid, similar to Coober Pedy, but those are rare in my experience.  Spencer material is unusual because that the fire lines are usually very flat, unlike the wavy fire lines in Australian opals.  That means they can be easily ground parallel to the fire line for triplet use.  This material cuts some of the best and brightest triplets known.  Some rare types even yield cat's-eyes and 3-ray stars.

My usual procedure is to choose the thickest and brightest fire line and carefully grind a face exactly parallel and very slightly into it.  That's the face you'll epoxy the black backing to.  I use basanite but some prefer black jade, black onyx or other materials.  It's good to leave some grinding marks to give the adhesive some "tooth" to grab onto.  The epoxy will function as a polish.  Trim the backing close to the outline of the opal as possible.

Now comes the tricky part -- sawing as close to the opposite side of the fire line as possible.  Once that's done you can again grind to the fire line and slightly into it.  Now epoxy on a piece of optical quartz and finish the stone to the desired shape.

This is the quick-and-dirty explanation.  If you have questions PM me.     
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lithicbeads

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Re: Question about cutting Spencer Idaho opal
« Reply #2 on: March 29, 2017, 09:10:52 AM »

Good advice. A legendary northwest opal cutter , Howard House , did find a thick solid  fire opal piece there many years ago. They were incredibly rare and it remains the only top quality  non doublet or triplet I have ever seen from Spencer.
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55fossil

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Re: Question about cutting Spencer Idaho opal
« Reply #3 on: March 29, 2017, 05:15:38 PM »

    I agree that triplets are the norm and best thing to do.  Thickness of the opal layer is not something to worry about though. After having the opal layer glued to a backing and cut thin it is time for the real work to begin. Much Spencer opal shows it's greatest fire when cut thin, very thin. Leaving some marks in the opal is okay but it still needs to be smooth. I grind to at least 600 grit before attaching to either backing or quartz cap.

   When you are looking at your rough there are many things to look for besides thickness of the opal layer. Brilliance and fire in a thin layer is better than a thick layer without much fire. If there are red lines in the layer it is most likely going to have red fracture lines through the complete layer of opal. Make sure and use the best glue you can afford! It needs to last for decades and never yellow.... ever. I use a 4 inch trim saw with very thin blades to cut out my veins of opal. Quite often Spencer has multiple layers in the same stone.

   Practice on some low grade pieces. It is not easy to cut those flat faces and keep an even thickness on the opal. Sounds like a lot but it is fun and gets easier.

good luck,  Neal
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ileney

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Re: Question about cutting Spencer Idaho opal
« Reply #4 on: March 31, 2017, 08:30:17 PM »

Many thanks for all the helpful advice and information.
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Orrum

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Re: Question about cutting Spencer Idaho opal
« Reply #5 on: March 31, 2017, 09:36:45 PM »

The matrix cabs very well and makes interesting attractive cabs!
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tkcaz

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Re: Question about cutting Spencer Idaho opal
« Reply #6 on: April 02, 2017, 07:25:58 PM »

Make sure and use the best glue you can afford! It needs to last for decades and never yellow.... ever.

Neal, what glue do you recommend?

TKC
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