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Newbie. What rock is this? Help!
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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Calisun




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PostPosted: Feb 14, 2019 14:00    Post subject: Newbie. What rock is this? Help!  

Hello everyone,

I am new to rock collecting, and have no background in geology. I was glad to find this forum, and look forward to sharing what I find, and learning more from you. found a couple of these rocks the other day in Southern California, and in spite of spending a lot of time trying to do image searches, I have no idea what they are. I am attaching the pictures of one of the two rocks. The sectional images are magnified (10x). Here are some observations: The colors seem to be black, yellow, and red with thin white lines. Some of the black circular patterns seem to have rings when viewed under 45x magnification. There seem to be crystalline areas. The rock weighs about 10 lbs. Any help in identifying the rock, and it’s charachteristics for reference would be much appreciated. Thank you.



D7629F37-9EE2-4C2E-84D8-FF6D750624CF.jpeg
 Mineral: SC
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GneissWare




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PostPosted: Feb 14, 2019 14:46    Post subject: Re: Newbie. What rock is this? Help!  

Exactly where were they found?
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Calisun




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PostPosted: Feb 14, 2019 14:58    Post subject: Re: Newbie. What rock is this? Help!  

Hi there. I found them while hiking a canyon in San Diego.
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cascaillou




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PostPosted: Feb 14, 2019 15:20    Post subject: Re: Newbie. What rock is this? Help!  

I'm not very good at identifying rocks, but this rock looks like it could have resulted from the metamorphism of limestone or sandstone.
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Calisun




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PostPosted: Feb 14, 2019 19:27    Post subject: Re: Newbie. What rock is this? Help!  

Thanks Casca. I am still trying to find out what these are. Searching images and such. No luck yet.
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cascaillou




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PostPosted: Feb 15, 2019 10:52    Post subject: Re: Newbie. What rock is this? Help!  

I'd suggest a few tests:

Isolate a small sample of the reddish material (it needs to be pure, I mean only the red material, without grains or veins of other stuff mixed in):

-using a sharp edge, can it scratch a glass mirror?

-then immerse a tiny grain of the same reddish material in a small quantity of hydrochloric acid: does it show effervescence (i.e. foaming/bubbling)? If not, does the grain dissolve after a day of soaking in the acid?
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Kevin Conroy




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PostPosted: Feb 16, 2019 12:53    Post subject: Re: Newbie. What rock is this? Help!  

Let me start by saying that I don't know much about fossils, and even less about the geology around San Diego, but your specimens remind me of some stromatolite fossils that I've seen.

However, I would take these to San Diego State University and show them to someone in the geology department. I'm sure that they have expert knowledge of the rocks, minerals and fossils of the area and would be able to help you. Do a search for "San Diego State University geology" to find contact info.
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Calisun




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PostPosted: Feb 16, 2019 15:08    Post subject: Re: Newbie. What rock is this? Update.  

Thank you all for the responses. I went back to the canyon, and found a fair bit of pink/purple rhyolite. I also polished (crudely), a small section of the rock, and the yellow bits seem like they are quartz (picture attached). I think this is quite possibly, highly-banded rhyolite. I think it looks pretty cool when polished!


8575B87E-672B-48BD-BE42-B0A3CC48CBBE.jpeg
 Mineral: Banded Rhyolite
 Description:
 Viewed:  523 Time(s)

8575B87E-672B-48BD-BE42-B0A3CC48CBBE.jpeg


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rweaver




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PostPosted: Feb 16, 2019 17:09    Post subject: Re: Newbie. What rock is this? Help!  

It would really help if you told us what canyon and where in San Diego County it is.
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Calisun




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PostPosted: Feb 18, 2019 14:26    Post subject: Re: Newbie. What rock is this? Help!  

I was trying to see if I could find a name for the canyon, but have not been able to. Its a small canyon, nestled in a residential neighborhood, in the city of San Diego, closer to the Mexican border. Hope this helps.
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Calisun




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PostPosted: Feb 18, 2019 23:34    Post subject: Re: Newbie. What rock is this? Help!  

Thank you Kevin, and Cascallou. I’ll look into taking it to the folks at one of the local universities. Casca, I don’t have any hcl, but I’ll keep you posted on what I find out. Justin, I doubt these folks here have any sinister intentions. I think everyone has been super nice and helpful, and if my discoveries lead to them finding more than I have, I’m all for it. Cheers!
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cascaillou




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PostPosted: Feb 19, 2019 00:29    Post subject: Re: Newbie. What rock is this? Help!  

If rhyolite is known in the area of the find, then that sounds like a good bet. I had ruled it out as I thought you were in sedimentary context, but this was actually one of my first thoughts, aside of some kind of marble, or quartzite based mixture.
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