Rock breaking requires a very solid and reliable base, such as an anvil or head of a sledgehammer. Because of molecular structure and complex irregular shapes of different rocks, rock breaking is unpredictable. Diamonds are split along fault lines. Choose rocks with fault lines that run in the direction of your break.
When striking a rock, do not lay it on the base and start hitting it. The rock must be lifted slightly from the base, so there are two support points. Since rocks do not flex, the lift only needs to be a small amount, never less than 1/4" or more than 1/2". Then strike the rock between the two support points, usually using a knife hand strike. You cannot hold back on the strike, so you should have a lot of experience with breaking before tackling a rock break.
Other materials include ice, stones, cinder blocks, tiles, clay bricks, coconuts, etc. Be careful when experimenting with other materials. Red bricks a will weaken if baked in the sun. Bricks may vary greatly in hardness due to the manufacturing process and the amount of sand (aggregate) in them. Generally, lighter colored bricks are easier to break.