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[R.U. Sirius]

I found this piece in a collection of mostly agates and jaspers I purchased recently, and am at a loss trying to pinpoint its locality and geological origin. White opal layers with green and red fire are found within white nodules, but the nodules are in this strange host rock made of glassy caramel material with millimeter-sized cream- and white-coloured globules. There is an occasional opalescent spot in the host, and there are also some rare patches of faint iridescence in the host rock (in the centre of the last image below), but I suspect this is simply due to thin fractures.

My uneducated guess is that this is one of the volcanic, not sedimentary opal-producing environments, but still couldn't find any decent matching examples online.

Any ideas?

[irockhound]

Spencer Opal from Idaho.  The color bands are normally laid down very flat just like water line agates.

[Felicia]

This looks a bit different than some purple and white potch opal I bought, but mine is from Mexico. It also fluoresces greenish. I got it to make something out of but now can't decide what. So collections grow.

[R.U. Sirius]

Spencer Opal from Idaho.  The color bands are normally laid down very flat just like water line agates.

That's exactly it, thank you! Lots of learning to do... I find this obsidian/rhyolite host rock as intriguing as the opal itself.

[vitzitziltecpatl]

What fun to find some Spencer in a random collection like that. Good for you!

Sounds like you might have gotten a few pieces. Even better! That would let you have a go at cutting a small cab. This material is most often used for doublets and triplets, but if you find a color bar that's thick enough you could cut yourself a real conversation piece. Not many solid Spencer cabs out there.

The thin, flat color bars in most of this rough will disappear in a split second on a grinder. Ask me how I know... .

Pick one with a bar close to the surface of the rough chunk and just have some fun. That way you can get a feel for it and maybe end up with a nice face-polished specimen.