It's actually not possible to make the hardest material also very tough. Harder materials can't yield to absorb energy and then spring back the way softer ones can. Their hardness means it requires less energy to break them. Take steel, for example. Bike-lock cables are made of very tough steel. You can't easily break a cable by hammering it. But compared with the high-carbon steel used to make tools, it's very soft, so it can be cut with a hacksaw.