A: Diamond is hardest along the octagonal planes. Any grinding in any direction in those four planes will be exceedingly difficult, but a drill bit grinds a circular path in only one plane, as it drills perpendicular to that plane. So long as you’re not attempting to drill perpendicular to an octagonal plane, the hardness affecting the drill particles will be less than maximum. The softest planes are the cubic planes, which would mean drilling directly into the table or culet, or in 4 places along the girdle plane, for most stones.

If you wished to drill into a harder direction, these days, you’d use a laser, which couldn’t care less about direction, and is a heck of a lot faster. Lasers are used not only to drill diamonds (usually for treatment of problem inclusions) but also as saws to saw diamond roughs in unusual shapes or difficult directions. In industry, diamond drawplates are widely used for drawing hard wire. These not only have to be drilled, but polished to very tight tolerances as well.

For the record, diamonds have been drilled in jewelry applications for quite a while now. I saw an antique piece at a Christie’s auction, some 15 years ago, which alternated cylindrical faceted beads of emerald with diamond rondell beads. Quite a piece, as I recall, and if I remember right, it was made around the turn of the century.