Precision in Motion

If you want to cut a smooth, rounded cabochon, you rely on a heavy, motorized arbor holding vertical grinding wheels. You hold the stone in your bare hands, relying on muscle memory and visual intuition to shape the dome.

But if you want to cut a brilliantly sparkling, geometrically perfect faceted gemstone, human hands are not stable or accurate enough. You need the assistance of a highly calibrated, micrometer-precise piece of engineering: The Faceting Machine.

A modern faceting machine is essentially a high-precision vertical milling machine mixed with a protractor. It allows the lapidary artist to lock a piece of rough gemstone into a mechanical arm and repeatedly lower it onto a spinning horizontal disk, cutting dozens of microscopic, flat planes (facets) at exact mathematical angles.

To understand the art of gem cutting, you must first understand the anatomy of the machine that makes it possible.

1. The Anatomy of the Machine

While there are several different styles of faceting machines (the two most common being the "Mast" style popular in America, and the "Jamb Peg" style traditionally used in Asia), the modern Mast-style machine consists of three primary systems working in unison.

The Platen and The Laps

At the base of the machine sits a heavy, precision-machined horizontal platter (the platen) driven by an electric motor. The cutter places flat, 8-inch circular disks—called Laps—onto this platen. Unlike cabbing, where you move the stone across different wheels, in faceting, the stone stays in the mechanical arm, and you constantly swap out the horizontal laps underneath it. Water drips continuously onto the lap from an overhead reservoir to keep the stone cool and wash away rock dust.

The Mast and Protractor

Rising vertically from the base is a thick steel pole called the Mast. Attached to this mast is the faceting head. The head can slide up and down the mast to adjust the height of the stone above the lap. Most importantly, the head contains a massive brass or steel Protractor. By turning a dial, the cutter can tilt the entire arm holding the stone to an exact, specific angle (e.g., 41.5 degrees) relative to the flat spinning lap.

The Quill and Index Gear

The Quill is the actual arm extending out from the protractor that holds the stone. Attached to the back of the quill is the Index Gear—a highly precise, toothed wheel. If the protractor controls the vertical tilt of the stone, the index gear controls the horizontal rotation of the stone. A standard 96-index gear has 96 perfectly spaced notches. By lifting a latch, turning the gear to notch "12", and locking it down, the cutter can cut a facet perfectly perpendicular to the one they just cut at notch "96". This is how the cutter achieves flawless radial symmetry around a round or oval gem.

2. The Doping Process

A faceting machine cannot hold a raw rock directly in its mechanical jaws. The stone must be attached to a Dop Stick—a small, precisely machined rod of brass or stainless steel, usually about two inches long.

The process of attaching the stone to the dop is called "dopping."

  1. The cutter selects a piece of raw gemstone rough and figures out which side will be the flat top (the table).
  2. They heat up a stick of specialized, shellac-based lapidary wax over a flame, or they use high-strength cyanoacrylate (superglue) and epoxy.
  3. They glue the flat side of the rough stone perfectly centered onto the end of the metal dop stick.
  4. The other end of the dop stick is inserted into the quill of the faceting machine and locked tightly into place. The stone is now securely suspended over the spinning lap, ready to be cut.

3. Cutting vs. Polishing Laps

The faceting process requires progressing through a series of different laps to achieve a final, mirror-like polish.

The Cutting Laps

The initial cutting is done using electroplated or sintered Diamond Laps. These are solid steel or aluminum disks embedded with industrial diamond grit.

  • A coarse lap (260 or 360 grit) is used to rapidly hog away excess rock and establish the basic geometric shape of the gem (the preform).
  • A medium/fine lap (600 or 1200 grit) is used to cut the actual, precise facets and bring all the intersection points together perfectly.

The Polishing Laps

A 1200-grit diamond lap leaves the facets flat, but they are frosty and opaque. To make the stone brilliant, the cutter must swap the cutting lap for a Polishing Lap.

Polishing laps do not have abrasive grit permanently embedded in them. Instead, they are made of softer materials like Tin, Zinc, Copper, or specialized phenolic resins. The cutter turns on the machine and manually applies a microscopic polishing compound (like 50,000-grit diamond paste, or a slurry of Cerium Oxide) to the spinning metal lap.

When the cutter lowers the stone onto the polishing lap, the soft metal of the lap grips the microscopic abrasive compound and scrubs it against the stone, melting away the frosty scratches and revealing a flawless, liquid-glass, optically perfect facet.

Operating a faceting machine requires intense focus, mechanical sympathy, and the ability to read a mathematical diagram. But watching a dull pebble transform into a blindingly brilliant jewel under your own hands makes mastering the machinery entirely worth the effort.