fossil id help

[3rdgenhound]

possible sea urchin? found near evidence of volcanic  activity,
lower portion was broken so i used lapidary grind and polishing
wheels grinding down just enough to level the surface and give a
clean view of this perfectly  crystallized interior that is a beautiful
clear fading to a amazing blue highlights and in the center about
a quarter inch down you can see preserved organs or bone of some
sort top is in perfect shape with beautiful blue crystal trim 


it is about a 3.5" diameter
and sits at about 3" height
weight unkown maxed out 200gram scale  

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[Joe Botting]

Welcome, and thanks for the query. I've got to say, it's quite hard to see much detail from the photos (they won't enlarge), and with these silicified structures the detail is what you need. You also don't say where it was found, or what age the rocks are (if you know that). There are siliceous concretions in the Carboniferous of South Wales, but I'm guessing you're from further away than that..? Also, what was the volcanic evidence? Do you mean hot springs, ash beds, lava flows...? They all result in different fossilisation pathways, so please do provide whatever information you've got.

So, what can we say from this..? There seems to a be a finely radiating structure on the flat surface, which is not normal for inorganic concretions, but is also incompatible with a sea urchin. There are fossils that have that sort of structure, such as various bryozoans, but if that's the case then there should be a trace of the structure through the blue chalcedony areas as well (unless it's completely recrystallised, which is possible). One possibility is something like an Araucaria cone - there are large numbers of silicified examples from Patagonia (associated with hot spring deposits), and I believe you get them in Utah (USA) as well. If you can post a more detailed set of pictures, then we might be able to get somewhere...

It's nice to see something completely different, anyway.

[3rdgenhound]

thanks for the info i will try to get some better pics for you today, it was found in the high desert of California near the edge of the Pisgah Crater lava flow a active volcano till the mid 1500's it was cut off by a major slip on the San Andreas fault which connects the north american plate and the pacific plate, it is not a Araucaria cone my old geology teacher told me the name of it a few years ago but i can not remember what he told me h also guested it is a rather young fossil he was guessing closer to 6000 then say 60,000 years old as that type of sea creature when extinct alot more recently then say trilobites found near that area i will definitely get some better pics uploaded today

thanks for your time and info,
respectfully,
Corey

[Joe Botting]

Mar 26, 2014 7:37:20 GMT -5 3rdgenhound said:

thanks for the info i will try to get some better pics for you today, it was found in the high desert of California near the edge of the Pisgah Crater lava flow a active volcano till the mid 1500's it was cut off by a major slip on the San Andreas fault which connects the north american plate and the pacific plate, it is not a Araucaria cone my old geology teacher told me the name of it a few years ago but i can not remember what he told me h also guested it is a rather young fossil he was guessing closer to 6000 then say 60,000 years old as that type of sea creature when extinct alot more recently then say trilobites found near that area i will definitely get some better pics uploaded today

thanks for your time and info,
respectfully,
Corey

Hi Corey. Yes, that puts a bit of a different perspective on it... the sediments in that area seem to be mostly Quaternary, so it could be very young indeed. That would make a marine fossil implausible - it's been a while since that area was under the sea. At the moment I'm leaning more towards this being some sort of geode rather than a fossil, especially as you do get some in that part of the world that include both chalcedony and radiating fibrous agate, as these things that people call 'thunder eggs':
stoneplus.cst.cmich.edu/thunderegg.htm
Most of these form in gas bubbles within lava flows, which fits with the sort of volcano you have at Pisgah.


I'm sure we'll be able to pin it down with some clearer photos, anyhow.

[3rdgenhound]

i added better pictures

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[Joe Botting]

Thanks for those, Corey - much clearer. I'm sure from this that it is one of those geodes that are colloquially called thunder eggs - not a fossil, but still very neat.