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What is this inclusion in Mexican fire opal


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36 minutes ago, Au Seeker said:

I'm no expert but I know that petrified wood can opalize so I think it's possible to have some petrified wood inclusions inside of opal and those inclusions look like wood to me. 

Thank you, but I do not believe that petrified wood or any fossil for this matter can occur in any volcanic rock. Yes, there is opal on petrified wood, fossil shells and so on, but opal could occur at different kind of minerals and that opal wood formed in a different way, not volcanic.

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Wow, did they really film this opal by putting it on the asphalt on a busy street at night? I can hear the cars passing by.

The opal shown in the video is not a fire opal. By definition fire opals have a red/yellow/orange base color. When an opal has a play of color, as the opal you show does, the proper name would be precious opal. A true fire opal, like the one Bill posted, when it has a play of color is known as a precious fire opal.

Nomenclature aside the opal in the video does appear to be of volcanic origin and was formed in a gray(?) rhyolite. It hard to tell in the video. The Mexican fire opals are found in a pinkish rhyolite.

Rhyolite is composed of several different minerals and those minerals can be altered quite a bit before they are opalized. The opal you show is bedded in porphyritic rhyolite with visible crystallization. The "wood" you see in the video is probably just an altered mineral crystal phenocryst that was later opalized. My guess, just looking at the general form and color, would be either small crystals of Sanidine (a high temperature potassium feldspar common to rhyolite),  a biotite mica book viewed from the edge (also common to rhyolite) or a limonite psuedomorph.

And guess is about all we can do with a photograph. Without destroying the opal to expose the "wood" to analysis we are left to our imaginations. If the opal came from a well known area where these are common inclusions there is some possibility that a mineralogist has researched this in the past but without more information about the locality and geology of the deposit even tracking down that possibility is a wild goose chase.

 

 

Edited by clay
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