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Found unusual rock, novice here


CHR

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Hi. I am a novice at identifying rocks. This was found in the Rocky Mountains. I cannot find any description in any info I have found that matches in any way. The rock has what looks like tubular streaks and at break looks like circular or ovals surrounded by a dark gray hard stone.  

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It is a fossil. It might be on limestone or some other sedimentary rock.

It is not really a rock if it is a fossil. It started out as a living organism. So it would be considered a mineraloid.

There is a name for rocks composed of fossils but I just can't remember the term. Someone may chime in and tell us. 

 

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1 hour ago, Bob(AK) said:

Bob, is the word you are looking for "geodized"

They are definitely geodized. But I was thinking there was a name for a rock comprised of (almost) complete fossil material. 

I tried googling a bit and didn't come across it. 

One of the members here used the term to describe a rock. I had never heard of the term before. I can't remember what the heck it was. 

Anyhow, the rock is indeed geodized fossils. I'm not sure what kind of critter they were but some sort of ancient sea life replaced with silica. 

I bet rocks like that would look sweet tumbled. It would be a lot of trouble getting the grit out of those voids. But the pattern would be pretty cool if you could get one rounded up.

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Very intriguing. I had considered many types of rock this might be, and ways It could have been made, but never could I have thought of “geodized” fossils. I had considered something volcanic, meteor, but no melted surface, I will study up on this form of rock. And many thanks for the info. 

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Try putting some warm vinegar on the sample to see if it fizzes.  If it fizzes its limestone.  If no fizz, then it is something else. 

I'm thinking this sample is a vesicular basalt (igneous rock which had lots of gases that left lots of bubbles holes once the lava cooled) where the holes (vesicles) were later filled in by quartz or calcite.  The filled in holes are called amygdaloids.

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Very intriguing. I had considered many types of rock this might be, and ways It could have been made, but never could I have thought of “geodized” fossils. I had considered something volcanic, meteor, but no melted surface, I will study up on this form of rock. And many thanks for the info. 

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