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Found these rocks over the weekend and would love to learn what they are. These were found in north east utah, also found what looks like solidified black bubbles in the immediate area. I tried to cut into these 2 rocks with my dremel, the only thing that cuts it is diamond.. 

Thanks in advance!

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On 9/7/2020 at 9:25 PM, Bedrock Bob said:

It looks like common opal to me.

Thanks bedrock bob. My first thought was chert, the rock shop I took it to said it was agate, but I just couldn't find anything that looked similar.. 

Here are a few more pictures. The 1st picture is under a uv light.

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Obsidian, a natural volcanic glass.

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One side looks glassy in the photos. The other side looks sugary. So it is going to be tough to tell. 

The main visual difference between agate and opal/obsidian is the size of the tiny crystals. Opal and obsidian are like glass with no (or very tiny) crystals. Agate has small but larger crystals 

The fractures indicate it is more glassy than an agate. It could easily be obsidian or opal from the photos. 

Both obsidian and opal are the product of volcanism. Both are different than other silicate in the way they form. Both can be about any color too.

You said you saw black bubbles. Those could be maerkenites, or obsidian nodules. But your specimen is not nodular. It looks more like opal to me. But it could certainly be obsidian. It will be very difficult to distinguish between the two based on just photos. And even hardness and specific gravity is so similar between the two minerals it would be tough to tell. 

One photo looks a lot like agate....where the stone is milky. The others look glassy, like opal or obsidian. So it is debatable.

Opal and obsidian are both basically natural glass. I think your specimen is closer to glass than agate based on the thin fractures. They are both lighter than agate and you can feel the difference easily when they are in your hand.

Opal often has a white cortex. It tends to look like a glassy bone on the outside edges. Obsidian weather's to perlite. So it tends to be velvety and porous on the weathered surfaces.

I hope that helps. It could certainly be either opal or obsidian based on just photos. 

If you find obsidian nodules in the area I would call it obsidian. If your area is extrusive volcanic geology with lots of milky, glassy veins I would call it opal. Either way you have one or the other.

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My money is on some variety of chalcedony (agate, jasper, chert, etc). The one photo looks somewhat like common opal, but it doesn’t in the other photos. 

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1 hour ago, d_day said:

My money is on some variety of chalcedony (agate, jasper, chert, etc). The one photo looks somewhat like common opal, but it doesn’t in the other photos. 

Doesn't seem to pass the agate test . . . Looks like chert or green jasper IMO.

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Thank you all for the feedback! I am planning on taking the rocks to another local shop where I can maybe put it under a high power microscope. I will be sure to let you know what I find!

Here are a couple of the black bubbles I spoke of. I also found a couple other rocks at the same spot, maybe there is just a mixture of minerals? See last 2 pictures.

 

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Do you have access to a short - long wave ultraviolet lamp? Sometimes those nodules can be fluorescent.

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12 hours ago, BMc said:

Doesn't seem to pass the agate test . . . Looks like chert or green jasper IMO.

"Many stones look like agates but aren't. For example, jasper and flint are closely related to agate but are opaque, not translucent. if a piece of rock completely blocks light from passing through it, then it is not an agate"

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3 minutes ago, BMc said:

"Many stones look like agates but aren't. For example, jasper and flint are closely related to agate but are opaque, not translucent. if a piece of rock completely blocks light from passing through it, then it is not an agate"

The first couple of pictures sure look like it could be translucent to me. The photos aren't taken with backlighting so I suppose we really don't know.

Obsidian, opal, and a whole host of minerals are translucent and aren't agates. And a whole bunch of agate is not really translucent. Often only in places. So the "agate test" is a bit subjective.

An translucent microcrystalline stone is an agate. A translucent cryptocrystalline stone is not. So just because a stone may be penetrated by light does not make it an agate. Conversely just because a stone is opaque does not mean it is not. 

We can argue about the line between jaspers and agate all day. At the end of the day it is the crystals that differentiate these rocks from amorphous and cryptocrystalline minerals like obsidian and opal. Not the translucency.

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14 minutes ago, BMc said:

Du gehst mir tierisch auf den Keks!

https://raregoldnuggets.com/?p=7052

"Agate and jasper are both varieties of chalcedony that are composed of microcrystalline quartz. Agate is a translucent to semitransparent chalcedony. If you have a piece that is semitransparent you will be able to hold a very thin piece up and see distorted or foggy images through it. If you hold a translucent piece up to a source of light you will see a small amount of light passing through the thin edges. If you hold it up to the light and pass your hand between the material and the source of light, your hand will block the light passing through the material. Jasper is an opaque variety of chalcedony. Opaque means that neither light nor images pass through"

 

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45 minutes ago, BMc said:

"Agate and jasper are both varieties of chalcedony that are composed of microcrystalline quartz. Agate is a translucent to semitransparent chalcedony. If you have a piece that is semitransparent you will be able to hold a very thin piece up and see distorted or foggy images through it. If you hold a translucent piece up to a source of light you will see a small amount of light passing through the thin edges. If you hold it up to the light and pass your hand between the material and the source of light, your hand will block the light passing through the material. Jasper is an opaque variety of chalcedony. Opaque means that neither light nor images pass through"

 

Indeed.

But we don't really know if this is translucent or opaque. Nor do we know whether it is microcrystalline or cryptocrystalline. 

So the "agate test" really means nothing here. 

Thus my point.

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Opal is a mineraloid and has no crystal structure - it is amorphous like glass.

Obsidian is a rock - not a mineral. It is composed of various minerals depending on where and how the felsic lava was ejected and cooled. Obsidian has no crystal structure but it sometimes does contain minerals that do have a crystal structure.

Hope that helps.

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My reference has to do only with a discussion of the difference between agate and jasper and how to possibly distinguish between the two. Perhaps my cataracts are getting in the way, but I don't see any translucent to semitransparent quality to the subject specimen. No quarrel with Micro or Crypto in the abstract. But Ipso Facto; the empirical evidence in re: to the subject specimen,(IMO), leans toward density and opaqueness.

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Microcrystalline and cryptocrystalline are synonymous terms, maybe you meant to say macrocrystalline Bob? Also for the OP, a hardness test would be most helpful, the first one posted looks like it could be Obsidian or common Opal but I think all of these might just be Chalcedony.

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Hey Dirtboi! You rascal

It is agate in my book. Those pics are a lot better. It is chunky. I can see dimension a lot better. And it is cryptocrystalline. It's not glass like opal or obsidian. 

It sure looks glassy in the one photo but it's not. 

2 hours ago, fuss said:

Microcrystalline and cryptocrystalline are synonymous terms, maybe you meant to say macrocrystalline Bob? Also for the OP, a hardness test would be most helpful, the first one posted looks like it could be Obsidian or common Opal but I think all of these might just be Chalcedony.

You are right about my terminology. I was screwing that up good.

Now please set me straight. Opal and obsidian are amorphous and not cryptocrystalline nor macrocrystalline? Glass and obsidian are vitrified and amorphous and opal is (mostly) amorphous?

I'm too lazy to look it up and it is a great discussion. I would appreciate it if you would correct my terminology again if im wrong.

Bob

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Last photo looks like a metal ore of some type.  Can you do a streak test and let us know what color the streak is?  Compare the "weight" of this sample with a similar sized chunk of agate you have.  Hold them at the same time, in different hands, and bounce them up and down in your hand(s).  Which one is heavier and give an approx ratio of the heavier one.  That is the heavier one is 1, 2,4,6... times heavier than the lighter one.  Better photos and close-up photos would be great.

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