The adjective for "sphere" is "spherical." You can say "a ball is round and spherical" but I don't like it because it is redundant. "Round" can mean spherical. (It can ALSO mean "circular.") In a geometrical context, the opposite of "flat" is "solid." Balls and blocks are solid objects. A block that is the same size in all three dimensions is a cube, and the adjective is "cubical." It is quite hard to describe things precisely in words because most of these words have overlapping meanings. For example, a coin and are both "circular," because the word circular can more than one meaning. This is how I would personally describe these objects: A coin is a flat, circular disk. ("Disk" tells is that it is NOT a ring.) A ball is a hollow, inflated sphere. A block is cubical. A table napkin is flat and square.