At first I gave up because there was no pattern and I like patterns... but I thought was more.
So after I completed the original pebbles I took a long hard look at it and funnily enough there was an
obvious paternity doubled:
(thats a pebble) ...> .
. .= 4 pebbles
. .= 1 unit squared.
Where as if it is a square you would have 8 pebbles and the same law applies for the rest of it.
I would definatly recommend doing pebbles first.
the amount the pebbles in which you use dos like this: 8 12 18 36
+4 +6 +18
I see no pattern in that but maybe other people will.