# rocks and pebbles

**URL:** <https://boards.straightdope.com/t/rocks-and-pebbles/42544>\
**Category:** Factual Questions\
**Created:** [November 24, 2000, 4:06pm UTC](https://boards.straightdope.com/t/rocks-and-pebbles/42544 "2000-11-24T16:06:58Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Colin\_Trainer](https://avatars.discourse-cdn.com/v4/letter/c/958977/32.png) [@Colin\_Trainer](https://boards.straightdope.com/u/Colin_Trainer)\
**Post date:** [November 24, 2000, 4:06pm UTC](https://boards.straightdope.com/t/rocks-and-pebbles/42544/1 "2000-11-24T16:06:58Z")

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when does a pebble become a rock? is there a size limit? and if you break up a rock how small do you have to go till it’s a pebble

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**Author:** ![August\_West](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/august_west/32/18069_2.png) [@August\_West](https://boards.straightdope.com/u/August_West)\
**Post date:** [November 24, 2000, 4:29pm UTC](https://boards.straightdope.com/t/rocks-and-pebbles/42544/2 "2000-11-24T16:29:14Z")

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> [@](#):
>
> when does a pebble become a rock?

A pebble is a rock. A pebble is usually defined as a small rock that has been rounded by the action of water.

> [@](#):
>
> and if you break up a rock how small do you have to go till it’s a pebble?

I would say less than 2" diameter, but you would also have to throw the pieces in a river for a few hundred years

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**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\
**Post date:** [November 24, 2000, 5:37pm UTC](https://boards.straightdope.com/t/rocks-and-pebbles/42544/3 "2000-11-24T17:37:15Z")

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A rock is a pebble when it can fit inside your shoe without you noticing until you start walking and apply pressure on it. :D:D

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**Author:** ![Yossarian](https://avatars.discourse-cdn.com/v4/letter/y/9de0a6/32.png) [@Yossarian](https://boards.straightdope.com/u/Yossarian)\
**Post date:** [November 24, 2000, 6:20pm UTC](https://boards.straightdope.com/t/rocks-and-pebbles/42544/4 "2000-11-24T18:20:56Z")

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The Standard Grain-Size Scale for Clastic Sediments (as devised by J.A. Udden and C.K. Wentworth) is the Geologist’s official word on what is and what is not a pebble. A pebble is any rock fragment (clast) between 4 and 64 mm in diameter. If it’s an inequant grain, you can choose to apply these dimensions to either the long axis or short axis as long as you do so consistently.

For future reference, rock fragments are divided into three main size categories:

Gravel (\>2 mm)  
Sand (0.062 to 2 mm)  
Mud (\<2 mm)

Subdivisions of gravel are:

Boulder (\>256 mm)  
Cobble (64 to 256 mm)  
Pebble (4 to 64 mm)  
Granule (2 to 4 mm)

Subdivisions of sand:

Very coarse sand (1 to 2 mm)  
Coarse sand (0.5 to 1 mm)  
Medium sand (0.25 to 0.5 mm)  
Fine sand (0.125 to 0.25 mm)  
Very fine sand (0.062 to 0.125 mm)

Subdivisions of mud:

Silt (0.004 to 0.062 mm)  
Clay (\<0.004 mm)

And, what the heck, we can keep going with silt:

Coarse silt (0.031 to 0.062 mm)  
Medium silt (0.016 to 0.031 mm)  
Fine silt (0.008 to 0.016 mm)  
Very fine silt (0.004 to 0.008 mm)

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**Author:** ![FloChi](https://avatars.discourse-cdn.com/v4/letter/f/58956e/32.png) [@FloChi](https://boards.straightdope.com/u/FloChi)\
**Post date:** [November 24, 2000, 6:47pm UTC](https://boards.straightdope.com/t/rocks-and-pebbles/42544/5 "2000-11-24T18:47:28Z")

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SO Colin Trainer were you expecting such a detailed reply?  
Good work there Pantellerite.

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**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\
**Post date:** [November 24, 2000, 10:04pm UTC](https://boards.straightdope.com/t/rocks-and-pebbles/42544/6 "2000-11-24T22:04:09Z")

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I’m related to a very young lady who calls them “pevvles.” If you get a whole lot of them, she says, you have “grabble.”

–Nott