Reinventing The Wheel: The OMF Concave Factor

(via Lapidary Journal, November 1997) Andy Oriel writes:

The story of the concave faceting machine is a revolutionary one—which could be said about any machine that revolves on several axes. But in the case of the OMF faceters, which stands for optically magnified facets, the description is justified.

Developed about 10 (20) years ago by Dough Hoffman of Polymetric Instruments, the machine is capable of cutting both concave and convex facets. Instead of grinding a stone against a flat lap as in conventional faceting, concave facets are ground against rotating copper and plastic cylinders of varying diameters, which are charged with abrasive compounds and move forward and back to prevent grooving.

“There are two main benefits to concave faceting,” says gem artists Mark Gronlund, slipping into teaching mode. “The first is the ability to cut new and unusual shapes. With flat facets you’re limited to shapes like rounds, ovals, marquise, and emeralds. But with the combination of flat and concave faceting, there are almost no limitations to shapes or girdle outlines.

“The second benefit,” he explains, “is an increase in the brilliance of a stone.” In a conventional stone, when light enters through the table and hit a pavilion facet, it is reflected in only one direction. By contrast, with a concave pavilion, which from the top of the stone appears as a convex surface, the light is scattered in many directions.

“Basically what we are doing is putting a bunch of magnifiers and diffusers into a stone, like little eyeglass lenses,” he says. “The convex surfaces focus light and the concave surfaces diffuse it.”

“Flat faceting is a three-step process with a rough cut, a final cut, and then polishing. With concave faceting, you still have to lay in all the flat faces first—the same first two steps. But then you transfer the interchangeable mast from the faceting machine to the OMF for an additional three to sometimes six more steps.”

If it sounds complex, it is. Suffice it to say that concave cut stones are several magnitudes brighter than normal, as if fire works were going off inside them; they appear to have many more facets than they actually have; and they tend to look slightly darker than otherwise, all of which have thrown the traditional colored stone industry a curve ball.

Less valuable stones, such as citrines and light aquamarines, are transformed from bridesmaid into blushing brides. A happy side effect of the phenomenon is that a cutter’s budget for rough is considerably stretched. And from the buyer’s point of view, large, precision-cut stones can be purchased for the price of mediocre, more traditionally precious ones.

There are imitations, however. Already dark stones, such as garnets and sapphires, tend not to benefit from the treatment. And small stones tend to become overly busy-looking like gliding a lily.