Stone is a block that is unminable, it spawns randomly in pockets simmaler to dirt, the only way to destroy this is with TNT. No other block may spawn inside a pocket of stone making each pocket completly solid, this can be rather annoying when mining as nothing but a sapper can get through it.
A slab of stone that has been blasted away at.
The stone slab in the picture has been chipped away at by a sapper to make a exact 3x3 cube, please note that it does not normally spawn like this as the normal spawns are usualy far larger and more odd in shape, the 3x3 cube is simply to get a good look at it.