Found this cool rock at lake Tahoe. It has 2 white rings around it that are so perfect I thought at first they were drawn. Now it lives in my Chia pet garden. Here's an additional pic.
this post was submitted on 30 Dec 2025
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Cool Rocks
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I would love to know as well. It's the main reason I posted this. Plus, it's a cool rock.
What's happened here is that when the pebble was part of the bedrock, it got shattered (probably by tectonic forces, bending the bedrock of the whole area). That left cracks all through it, at all sorts of angles.
Those cracks became spaces where water could get in, and it carried something which crystallised over the course of many years and filled the cracks.
Later the pebble broke out of the bedrock, and got eroded round. The white rings are actually flat (or at least, flattish) layers of white which cut all the way through the pebble
Whoa... So if I cut this rock open right there it would be all the way through?
Should be, yeah
Earthquake probably. The surface was that white layer then quake and now the top layer is sideways and a new layer of white is deposited. Not a geologist.
If it has a fault inside it's a very cool rock, so I'd be cautious about the possibility.
What do you mean a fault in the rock?
In geology, a "fault" is a place where rock has broken apart and the parts have moved, so the two faces don't match up any more.
This would actually be "jointing", where it's broken but there was no movement