A diamond is 58 times harder than the next hardest mineral on earth, corundum, from which rubies and sapphires are formed. It was only during the 15th century that it was discovered that the only way to cut diamonds was with other diamonds. Yet, diamonds are brittle. If hit hard with a hammer, a diamond will shatter or splinter.
Diamond is not an element, it is a crystalline form. It is not the strongest material know. It is the hardest. Diamond cutters use copper wedges to "cut" diamonds. They are actually breaking the diamond along cleavage planes rather than cutting it. The copper wedge would not put a dent in soft steel, but because it deforms slightly around th diamond, it can apply the impulse from the diamond cutters mallet more uniformly along a line selected along the cleavage plane than could a steel wedge.
. Think of a steel structure, like a light pole, along the coast being sheared off by a wave. The water is not stronger that the steel, but it can apply more force than the steel can withstand.