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
159 points (100.0% liked)
Cool Rocks
255 readers
161 users here now
A place for:
- Sharing cool rocks you found
- Discussing cool rocks
- Memeing about rocks
- Cool rocks
- Minerals are cool, too
Be gneiss to each other.
I need another mod! This place is building up fast. DM me if you'd like to help tend to the rocks.
founded 3 days ago
MODERATORS
you are viewing a single comment's thread
view the rest of the comments
view the rest of the comments
Hmm, it's very interesting how they're angled relative to each other. Anyone have ideas on how that could happen?
I think it’s basalt with quartz veins.
https://www.wikipedia.org/wiki/Vein_(geology)
Likely the basalt fractured and then quartz filled in the cracks.
I thought of that too, but if so it's remarkable the junction of the cracks ended up right at the surface of the rock.
Not impossible, I'd take that before a fault like someone else suggested, but if there was a way it deposited after to stone was out that would explain it really well - one ring formed, then the rock got knocked out of place and another ring of accretion formed at the new boundary.
They aren't rings, but layers. There were likely many more cracks that filled and created other layers that later broke apart and eroded to form this rock.
The cracks in the basalt weren’t parallel. They filled in at the same time. They weren’t (probably) horizontal. Much later, after it was filled in, it was broken again. Then water (river or ocean) smoothed it to be round.
edit: They’re not rings, they’re through the whole rock in a plane. “Layers” isn’t right either. The filled in part wasn’t horizontal when deposited. The quartz crystals grew in the cracks whichever orientation they were. So the single stone you have is really three basalt pieces stuck together with quartz “glue”
I did find this on the shore but it was lake Tahoe at around 7,000ft elevation. Could it still be from the ocean in that case? You guys seem to know what you're talking about and it's terribly interesting. I always just thought, "cool rock" but I love hearing all these theories.
Could be from one of the streams leading into the lake. Could be many, many years of the small but present waves of the lake hitting the shore. Or could have been created when the Sierra Nevada were closer to sea level.
Nice. Tahoe gets huge waves from time to time. Most of my cool rocks come from that area.
Waves were my top theory.
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