I've done this though not in C2 but here's how I accomplished it loosely based on the original game logic.
First decide which objects you want the boulders to fall from. I selected boulders and gems. in classic BD they call these objects round.
assign a boolean var to each rock instance eg isRound and set to false by default.
Now you'll need to acquaint yourself with the "overlapping at offset" event.
Say you have a 3x3 grid and your rock is on the middle square.
On start of level for each rock:
Check if it's overlapping a round object at offset x=0, Y=(1 X tilesize)corresponding to square 8.
If it is then set the isRound var to true. The rock can now roll
Make a timed loop
System every 0.2 seconds
System for each rock
add a second condition so it only checks rocks that you permit may fall.
if isRound is true
Check if it's overlapping an object on the square to your left.
if it is then check if it's overlapping an object on the square to your right.
If it isn't overlapping anything on the left ie the square is empty then check if it's overlapping anything in the square left one, down one if it isn't then fire the "Roll" function) if it is overlapping at either of those two points try the same thing on the right ie is X overlapping at X + tilesize, Y=Y then X + tilesize, Y + tilesize.
Back to the square of nine Rock is on 5 you check 4 then 7 if both empty roll rock left if either occupied check 6 then 9 if both empty roll right if either occupied then no roll possible but now is a good time to update the Rock vars as something may have changed since the last time So add an Else:
Check if it's overlapping a round object at offset x=0, Y=(1 X tilesize)corresponding to square 8. (As at beginning)
Game has moved all moveable instances and updated all instances to see if they are potentially movable.
Roll function can be simple movement 1 x tilesize left or right and let gravity do the rest bullet works best.
Use platform behaviour to detect if a rock is falling or has landed and toggle the isRound var otherwise you'll have rocks dropping off objects you don't want, round only!
As in BD the same movements can be applied to the diamonds to achieve the correct drop effect for those too.