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Is this a rock?

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I found this in the bed of the Mad River in northern California.  My guess would be that its some sort of quartz deposits on a sandstone, but that is just my novice guess.  Those little bubble like forms have the white inside and that is why I thought they might be quartz.  It is about 8 1/4 inches.  I liked the shape so brought it home to add to my collection but would like to know a little bit more if anyone can help.

asked Aug 29, 2016 by April (180 points)

1 Answer

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Rocks and sediment and water combined to make a rock which is called a concretion.
answered Aug 30, 2016 by Weasel (52,110 points)

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