Diamonds are excellent for cutting some materials, e.g. ceramics or some non-ferrous alloys, but they are bad for cutting metals that are good at dissolving car

Diamonds are excellent for cutting some materials, e.g. ceramics or some non-ferrous alloys, but they are bad for cutting metals that are good at dissolving carbon, e.g. iron, cobalt and nickel alloys. So for the iron alloys, which are the material most frequently processed by machining, diamond is not suitable. Other hard crystals, like alumina a.k.a. corundum, are much better for this purpose, even if they are less hard than diamond.