Commandblock commands?

Hi

Does anyone know if there's a minecraft command with which you can, for example, convert foliage to quarry stone?

You can clear all cobblestones first and then give the cleared number of leaves. Just look at a Minecraft Wiki

Yes there's. At position you enter the coordinates of the block, at block ID the ID and possibly also the state of the block. With replace all blocks available at the specific location are replaced with the block ID.

Otherwise, the Minecraft Wiki is a very helpful page for questions about commands.

/ setblock position (X Y Z) block ID replace