I just watched a small History Channel video on the cutting of a diamond. Look at this site. I always thought that diamonds are cut in such a way as to go with the grain of the crystal, a result of the way carbon atoms are linked together. But it just isn't so. The diamonds are ground down to the half conical shape (this is called bruiting), and then they are ground into 57 facets that give the stone its fire.

How do you bruit a diamond? In the video, it looked like they used a small lathe. What was grinding it? It looked like it was being ground by another diamond. It makes sense. An object of any one hardness (measured in mohs) can scratch another object made of the same material. Diamond is the hardest substance next to carborundum, and thus you would only be able to scratch a diamond with a diamond (or carborundum).

Then to make the facets, they use a turntable as a kind of sander, and polish the stone. It is the polishing that makes the 57 facets.

I've thought about it, and diamond really doesn't have a molecular structure that is weak on any one side. Graphite, being another form of pure carbon, forms 2-dimensional hexagons, and they can be cleaved into sheets. the hexagonal molecules hang together in one plane, but the connections between planes are very weak. This is one reason why graphite is a good lubricator.

Carbon atoms in diamond, on the other hand, are configured in a tetrahedral formation. Four carbons mark the apices of the tetrahedron, and they are connected to a fifth carbon which is at the center. Here is a clear model of a diamond's structure. Since there is no weak spot, there is nothing to cleave, like with sheets of graphite.

So those 57 facets must be ground down instead of cleaved. 

It really takes the air out of the sails of this one car commercial I saw back in the 70's. I can't remember what car it was, but it was supposed to have such a smooth ride to it, that the well-dressed jeweler in the back seat could make a proper cut in a diamond without funking it. The jeweler placed some sort of wedge tool on his diamond and tapped it with a hammer. Voila! he holds up a properly faceted diamond! What a farce! 

A diamond is really nothing to take so lightly! Nor will it shape so easily! It takes 7 weeks to polish a diamond, and they have to be done by hand. Computers are only used as gauging and estimating tools. It is the technician who eyeballs the final faceting.

Amazing!