# What kind of rock is this? Why is it so round? **URL:** \ **Category:** Factual Questions\ **Created:** [December 5, 2019, 12:45am UTC](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386 "2019-12-05T00:45:40Z")\ **Posts on this page:** 6\ **Page:** 4
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [December 7, 2019, 5:13pm UTC](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386/61 "2019-12-07T17:13:57Z")
> [@msmith537](#): > > Seriously, it looks like any old rock you might find in or around streams and rivers and whatnot. And your qualifications to judge are…? Placed against my actual geology and archaeology degrees, that is? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [December 7, 2019, 8:57pm UTC](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386/62 "2019-12-07T20:57:08Z")
> [@MrDibble](#): > > And your qualifications to judge are…? > > Placed against my actual geology and archaeology degrees, that is? I presume this means you’re never going to dispute anything I post on biology then?😉 ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [December 8, 2019, 6:19am UTC](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386/63 "2019-12-08T06:19:53Z")
> [@Colibri](#): > > I presume this means you’re never going to dispute anything I post on biology then?😉 This is about the “Bird Guy” dig, isn’t it :D? You know I wouldn’t dispute anything you said _without a good basis_ (which includes two undergrad years of bio subjects). I’m not claiming to be the ultimate Dope authority on geology or PNW stone tools, far from it. There are others here who are probably much more expert in both than me. I’m just saying I 'm far from a layman in rocks and archaeology, and I was wondering if **msmith537** had a basis for argument. It doesn’t seem they do, beyond “seeing lots of rocks around…and whatnot” ---
**Author:** ![pmwgreen](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@pmwgreen](https://boards.straightdope.com/u/pmwgreen)\ **Post date:** [December 8, 2019, 6:06pm UTC](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386/64 "2019-12-08T18:06:59Z")
> [@The\_Stafford\_Cripps](#): > > In my opinion there is no reason to think that the stone’s shape isn’t natural. You can find lots of very rounded stones like it on a pebbly beach, especially when the rock isn’t stratified. It might have been carried to the find spot by humans, but I’d want to rule out local morraine as a source. “pebbly beach” is probably about it. Transported by glacier, rounded by waves, selected by someone for roundness out of all the stones on the beach and then discarded near a path in the woods when they got bored. Or it could be aliens. Anyway, it does give new meaning to the phrase “You don’t know where it’s been.” ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [December 10, 2019, 8:42pm UTC](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386/65 "2019-12-10T20:42:45Z")
Agree with others that the mineral appears to be granite. Here is a different way it _might_ have become round and polished: [Gastrolyths](https://en.wikipedia.org/wiki/Gastrolith) are stones/sand swallowed by birds that mechanically pulverize food within their gizzard.(AKA craw) If birds are not dinosaurs, then they are the closest living things, and it turns out that large gastrolyths have been found with some dinosaur fossil remains. At least some dinosaurs had gizzards like modern birds. But big enough to hold actual rocks rather than just sand or gravel like tiny modern birds. I have a dinosaur gastrolyth somewhere around here. It is slightly larger than a golf ball, light colored stone, nearly white…limestone I’d guess. It was given to me by my friend Peter, who got a half dozen or so from another friend who found around a half-bushel of them while excavating for a driveway slab somewhere in Florida. They are too common for the museum or even the U of F paleontology department to take an interest in. They are just a little too cool to throw away, but not useful enough to be worth keeping around. So the finder gave a sack full to all his friends, one of whom passed one on to me. For some reason they each have one side that is flattened, and in some cases concave…they are OK for paperweights, if a little small. If that stone is very round, polished, and especially if one spot is flattened or even concave, it might well be a dinosaur gastrolyth. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [December 10, 2019, 9:20pm UTC](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386/66 "2019-12-10T21:20:41Z")
> [@Kevbo](#): > > They are too common for the museum or even the U of F paleontology department to take an interest in. No, the museum and the U of F are just tired of having people bringing in every vaguely rounded rock that they find and claiming that it is a gastrolith. You should read this part of the Wiki you linked: > [@](#): > > Geologists usually require several pieces of evidence before they will accept that a rock was used by a dinosaur to aid its digestion. First, the stone must be unlike the rock found in its geological vicinity. Secondly, it should be rounded and polished, because inside a dinosaur’s gizzard any genuine gastrolith would have been acted upon by other stones and fibrous materials in a process similar to the action of a rock tumbler. Lastly, the stone must be found with the fossils of the dinosaur which ingested it. It is this last criterion that causes trouble in identification, as smooth stones found without context can (possibly erroneously in some cases) be dismissed as having been polished by water or wind. Scientists don’t not want your “gastrolith” because they are so common–they don’t want it because the chance you have one is approximately 0.00000000000274 %. [Previous page](https://boards.straightdope.com/t/what-kind-of-rock-is-this-why-is-it-so-round/844386.md?page=3)