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Post by vegasjames on Jan 31, 2019 4:12:58 GMT -5
That is some super material you have collected there James. I had not heard the term "common opal" before. Will have to research it now. Thanks for sharing. Pretty simple. Any opal that has flash is called a precious opal. Any opal without flash is called common opal. The of course there are the numerous subtypes of each.
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Post by greig on Jan 31, 2019 11:15:06 GMT -5
Now for a dumb question. If you find one of these rocks, how do you know the difference between opal and agate? For example, I would have called the green ones "moss agate".
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Deleted
Deleted Member
Member since January 1970
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Post by Deleted on Jan 31, 2019 12:50:58 GMT -5
Now for a dumb question. If you find one of these rocks, how do you know the difference between opal and agate? For example, I would have called the green ones "moss agate". Not a bad question at all. My own first reaction is based on appearance - which is far from fool-proof - but on a freshly broken surface opal has a waxy luster and feel; agate has a harder, shine and slicker feel. Agate is generally a bit tougher/harder than opal on a scratch test. Opal and agate can comingle in the same rock, and some agate contains enough minerals beside silica that it can look and feel similar to opal, so appearance alone is just the first step to ID something from an unfamiliar area. In addition, slag and a few other possibilities can appear to be opal at first glance. To nail down a positive ID, further testing is going to have to be done (RI and SG at a minimum).
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pizzano
Cave Dweller
Member since February 2018
Posts: 1,390
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Post by pizzano on Jan 31, 2019 18:04:50 GMT -5
Examples of "common" Opal within a matrix of Basalt/Andesite can be seen here....:
forum.rocktumblinghobby.com/thread/86284/opal-re-visited
Those examples are not near as colorful as what James has displayed........there are a few bands of "precious" mixed within the stones shown, but most of what I polished is a creamy white and yellow mix..........very common finds in the Mojave Barstow area.
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Post by vegasjames on Jan 31, 2019 18:56:49 GMT -5
Now for a dumb question. If you find one of these rocks, how do you know the difference between opal and agate? For example, I would have called the green ones "moss agate". Not dumb at all.
Opal has a different look and feel. You will see a very shiny, waxy surface and the feel is much slicker than with chalcedonies such as jasper and agates in most cases. An exception are the porcelain jaspers like I have found in Northern Nevada. They are super slick but also have a higher density.
Opal does occur with chalcedonies for a simple reason. Opal starts as a silica gel. As the silica molecules move closer together they start to squeeze out water. When enough water is squeezed out you have opal, which has various percentages of water. When all the water squeezes out the opal converts in to chalcedony. I have found some pieces that are in the transition as well. Not quite as slick as opal but not quite to the feel of a chalcedony yet.
Hardness is less than the chalcedonies as well but can get close up to a hardness of 6.5.
Density also varies due to variations in the water and aluminum oxide contents, but opal specific gravity is still lower than that of the chalcedonies.
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goatgrinder
spending too much on rocks
Make mine a man cave
Member since January 2017
Posts: 368
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Post by goatgrinder on Jan 31, 2019 19:32:28 GMT -5
Wonderful discussion.
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hummingbirdstones2
fully equipped rock polisher
Vince A., 1958-2023
Member since August 2018
Posts: 1,461
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Post by hummingbirdstones2 on Jan 31, 2019 22:13:36 GMT -5
Thanks for posting the photo of the gray one. We got a chunk of that in an old collection and didn't have a clue where it came from.
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Post by Pat on Jan 31, 2019 23:21:58 GMT -5
Thanks for posting ! I thought common opal was always light pink! Looks like it can be any color. Is that right?
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Post by vegasjames on Feb 1, 2019 1:58:43 GMT -5
Thanks for posting ! I thought common opal was always light pink! Looks like it can be any color. Is that right? Yes. I have found common opal in various shades of purple from a light lavender to purple so dark I originally thought it was black, light to dark green, pink, salmon, gold, blue-gray, red, yellow, white and butterscotch colored.
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