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Post by tkrueger3 on Aug 27, 2010 13:32:34 GMT -5
I haven't been able to find any photos of what an opal-bearing rock would look like - particularly one containing white potch, or common opal. I have a fist-sized rock that I made 2 cuts on this morning, and I want to understand what I'm looking at. Here's a pic of the 2 cuts: So, the white stuff - might that be "common" opal, or "potch"? I can't see any sign of color in it at all. If it's not opal, then what is it? Sorry I'm so lame at this geologic taxonomy stuff! Thanks for any help. Tom
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jcinpc
has rocks in the head
Member since April 2009
Posts: 722
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Post by jcinpc on Aug 27, 2010 22:58:11 GMT -5
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rallyrocks
fully equipped rock polisher
Member since November 2005
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Post by rallyrocks on Aug 31, 2010 15:03:46 GMT -5
"Opal bearing rock" varies quite a lot depending on the location of origin, in Nevada you can find two primary occurrence types- opal bearing clays where the opal is often found as limb casts or opalized woods and opal bearing basalts where opal forms in small pockets and bubbles in much harder basalt- Idaho's Spencer opal forms in obsidian- but the color is found only in thin layers within the white masses. Oregon's opal butte locality yields opal filled thundereggs. Australian opal also varies a lot, from veins in clay, bounded by gray potch, to the boulder opal which is small pockets in reddish brown ironstone. "Potch" usually refers to any opal that does appear to have any clarity or opalescence, but that does not preclude it having some color, and it can vary from white to black and again depending on locality- just about every pastel color in between.
Your fist sized rock looks to my eye in this picture more like an agate than an opal, nice stuff regardless...
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jcinpc
has rocks in the head
Member since April 2009
Posts: 722
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Post by jcinpc on Aug 31, 2010 19:19:09 GMT -5
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Post by tkrueger3 on Aug 31, 2010 20:22:10 GMT -5
Thanks for the explanation - it makes a lot more sense to me now.
Tom
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Post by johnjsgems on Sept 28, 2010 20:14:32 GMT -5
I generally hear "potch" as the softer matrix material. Common opal is opal without any "fire". It will look much glassier and wetter than agate or jasper.
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Post by susand24224 on Oct 5, 2010 1:03:55 GMT -5
I just read in an "advanced cabochons" book (an old one) that precious opal is an opal that is translucent, regardless of whether there is fire. Here (Washington State) translucent and opaque occur together. I had always thought that precious opal is that with fire, as John stated, so I guess there may be multiple definitions.
Susan
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Post by johnjsgems on Oct 10, 2010 20:56:17 GMT -5
Precious priced opal always has fire. A lot of translucent opal with color is marketed as "jelly opal" .
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