[Bernard Marx]

Now you made me go do some work. I was confused because I know CVD techniques are being used to grow gem diamonds and polycrystalline materials are almost impossible to polish. It turns out there are several different techniques, most yielding polycrystalline diamond for industrial applications.

But a single crystal diamond substrate can be used as a template for further single crystal growth in which the synthetic grows with the same crystallographic orientation everywhere on the substrate. (I don't know how they get those substrates but they do). If it's grown to great enough thickness the diamond produced this way can be polished into faceted stones. I just couldn't recall the details.

[null and void]

Ah. My interest was from an engineering materials standpoint. Thanks for the additional information!