Trapped Cracked Blocks that can break..

Trapped Cracked Blocks!  Put a cracked block into the furnace for a second go around it converts into a TCB.  TCBs would have a special durability which would run down if you push up against it (very slowly), stand on it (moderately quickly) or jump up and down on it/fall on it unless there is another more solid block under it.   It would be great for creating traps... it still gives the players a clue that it might be broken by looking cracked.

When a TCB breaks it turns into a rubble block.  Rubble blocks are small piles of cubes (since it's minecraft) of whatever material the block was originally and are about the height of a campfire and are affected by gravity.  They make a falling rock noise when they fall.  Small mobs (baby animals, chickens, parrots, cats and dogs, and spiders) don't trigger them.  Stacks of TCBs would be prone to collapsing if someone breaks the top one, since the player will fall a little bit when that breaks landing them on the TCBs below harder.

Multiple stacks of rubble can fall into the same place, making it look more cluttered (sort of like adding turtle eggs to a square.)  If you break the rubble block you don't get anything, just a cleared space (unless you use a silk touch tool, in which case you can pick it up piece by piece.)  Rail and other redstone items placed on it break when the TCB breaks.  Minecarts running over rails placed on TCBs can break them at about the same speed as jumping on them would.  TCBs have a blast resistance of zero.