What causes the round shape of pebbles?

While walking along a beach recently, I noticed that most of the pebbles, whatever their size, tend to have an ellipsoidal shape, often egg-shaped with two small axes of similar length, and sometimes flat, with all of the axes different. ... I would expect all pebbles to tend to the spherical, the shape which takes the least energy to roll and to which particles will tend as their "corners" are eroded. Or are all pebbles still on their way to becoming spherical?
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On entering the sea, each type will undergo the same processes, turning the first type into elliptical forms and the second type into flatter forms. Those starting as ellipses will become more elliptical.
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The mechanism causing this is essentially a single force-the sea-pushing individual pebbles over the top of more pebbles. The steady state of a rotating body will be about its major axis, so that the moment of inertia is minimised.
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We have smooth, deep, cylindrical potholes in the sandstone beds of local mountain streams. These produce smooth, well-rounded stones, sometimes startlingly spherical. In contrast, stones in open stream beds take on smoothed irregular shapes.

Andre said:

This seems to be a very dumb question. Just google and you'll find http://www.newscientist.com/lastword/article.jsp?id=lw618



So the answer seems to be polishing by water. So why bother us with this thread?

Because so many things in geology are totally different.
Why are pebbles rounded?

I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean lay all undiscovered before me.

Sir Isaac Newton,

Pebbles are rounded in the course of many million years because they:

a - are polished by water
b - are sand polished by winds
c - are tumbling and expose different sides to the erosive effects
X d - grow that way
e - a combination of a. and b. and c. is true

But how can you tell whether it formed from the inside out ("grew") or the outside in? If it stated life as a bubble in viscous rock mass which cooled quickly, and later had lots of nice, hot, mineral-laden water flow through it,

'depositing' lovely crystals as the temparature cooled, wouldn't it also end up as round, with more-or-less concentric layers?

The Moeraki Boulders, geological curiosities each weighing several tonnes, and measuring up to 4 metres in circumference, lie on Hampden Beach much as if a giant, weary of his play, has walked away from a game of bowls - so much so that the early whalers named them The Ninepins and the location, Vulcan's Foundry. The spherical, grey, septarian boulders were formed on the sea floor about 60 million years ago by the gradual accumulation of lime salts around a small centre. Their shape is thus caused by chemistry and not by wave action.

Manganese nodules are pebbles or stones about the size of walnuts that are built of onionlike layers of manganese and iron oxides. Minor constituents include copper, nickel, and cobalt, making the nodules a potential ore of these valuable elements. Mining of manganese nodules has been the subject of study and experimentation since the 1950s (see below Economic aspects of the oceans: Sources of minerals and other raw materials).