Re: If diamonds are so very hard, why can they be shattered with a hammerblow?

The diamond used on the edges on drill bits to cut through tough materials is polycrystalline diamond. This means that in terms of its crystal structure, the bonds are oriented in random directions.

With a diamond crystal, where each bond is oriented perfectly in accordance with its crystal structure, there exist cleavage planes. Most minerals have cleavage planes, which are planes along which the bonds between atoms are relatively weak. Because of this relative weakness, the bonds in this directions break rather easily, and if you hit a diamond the right way, it will shatter, but the planes of the shattered pieces are smooth.

So, back to the drill bit diamond I mentioned in the first paragraph. Because the bonds are oriented randomly, this kind of diamond is not subject to cleavage.