In order to determine whether pebbles from a beach will make a good form of money, we have to look at the properties that an object must have in order to be useful as money. There are generally said to be six properties that an object must have. They are:
Portability. You need to be able to carry money from place to place.
Durability. You cannot have money that will fall apart when people handle it, carry it, and otherwise use it from day to day.
Divisible. If you can’t divide money, you can’t buy things that are worth less than one unit of whatever sort of money you are using.
Stability in value. Money needs to keep its value. It can’t do things like rotting or evaporating.
Relative scarcity. If money is not scarce enough, it will have little value.
Acceptability. Most or all people must be willing to accept a thing.
Of these, the biggest problem with pebbles from the beach is scarcity. You could carry pebbles (though carrying many of them would be a problem) and they will not wear out. They are pretty small, so they could be used to buy most things. They will not rot or otherwise disappear. The problem is that pebbles are way too common to use for money. If anyone can go off to a beach or other such place and dig up a lot of a substance, it is not very useful as money.