Most facetable black diamonds are industrials. They’re a maze of fractures and alternating growth directions, and pieces tend to fall off during cutting. Because of the alternating crystal growth, cutters also encounter different directions of hardnesses on the same facet. “When you’re talking about black diamond, it’s really a conglomerate,” says natural colored diamond dealer Alan Bronstein. “The more you cut, the more little things keep falling off the stone.”
Nizam Peters, director of the American Institute of Diamond Cutting Inc. in Deerfield Beach, Fla., says there are two rules of thumb when cutting black diamond. “If the stone has a good orderly arrangement of growth, there will be only a little resistance, since there are still carbon inclusions, but normally it will be straightforward cutting. However, most black diamond has a great deal of resistance. They are highly knotted, and it takes a special wheel to polish these stones.”
The special wheels, according to Nizam, are impregnated with diamond and have diamond bonded to their surfaces. “It’s a coarser powder,” he says. “A powder of larger grit size, for larger ripping.” Most standard wheels have powder glued to the surface, notes Nizam. And some wheels have both the standard glued powdered surface as well as an area of impregnated coarser grit. “You’re never going to get as good a polish on black diamond as you would on a normal piece of rough,” Nizam says. “You will get drag lines, but it will be nice enough to be presentable.”
Impregnating the wheel can double the usual cutting time, depending on the knots. “It’s more like two to three weeks, provided you have the right wheels,” Nizam says.