A cut diamond is a piece of geometry — every facet placed by a computer to bounce light back to the eye. A raw diamond is a piece of the earth. We prefer the raw, not because it is cheaper (it often is not) but because no two are alike. The setting has to be made for the stone, not the other way around.
That irregularity is a feature, not a flaw. A raw diamond is the only diamond whose shape you can read with your thumb. It catches light in fewer directions than a cut stone, but the directions it does catch are yours alone — no one else has the same facet angles.
Raw diamonds are not for everyone. If you want the rainbow fire of a brilliant cut, raw will disappoint you. If you want a stone that reads as found — geological, particular, slightly defiant — a raw diamond will reward you for years.