Hello,

Rock is the generic term given to objects like stones and pebbles which are made up of rock material.

Stone is rock material which can big or small.

Gravel is the weathered rock which is made by sea or water bodies which is found mostly in gravel pit or river river bank.

Boulder:- they are the big size rocks which has covers space over a foot. It can be found in mountain or hilly areas. They are very very big in size.

Pebbles are inches size stones or small stones.

Stone, gravel, Boulder and pebbles are different kinds of rocks which vary by size and location they are found out.

Hope it helps!