A faceting machine is a precision lapidary instrument designed to cut and polish flat, geometrically exact facets on transparent gemstones. By controlling two independent variables — the angle at which the stone meets the grinding surface, and the rotational position of the stone around its vertical axis — a skilled operator can reproduce any faceting diagram with a degree of accuracy impossible to achieve by hand. The faceting machine is the defining tool of modern amateur and professional gem cutting, and its development over the twentieth century transformed faceting from a largely industrial craft into a discipline pursued with scientific rigour.

Principal Components

All faceting machines share a common architecture, regardless of manufacturer or price point.

• Lap: A horizontal, motor-driven disc — typically 150 to 300 mm in diameter — onto which the stone is pressed to grind or polish each facet. Laps are interchangeable and are made from a variety of materials: cast iron charged with diamond powder for cutting, copper or tin for pre-polishing, and specialised surfaces such as ceramic, typite, or oxide-charged felt for final polish. The lap rotates at a controlled speed, usually between 100 and 500 rpm.
• Mast: A vertical post, fixed to the machine's base, along which the cutting head travels up and down. The mast provides the stable reference axis from which all angle measurements are made.
• Quill: A cylindrical spindle that holds the dop stick — a metal or wooden rod to which the rough stone is temporarily cemented with dopping wax or two-part epoxy. The quill slides within the cutting head and can be locked at a precise height, controlling the depth of each facet cut.
• Angle adjustment: The cutting head pivots around a horizontal axis, allowing the operator to set the angle between the stone's facet and the plane of the lap. On most machines this is read from a protractor scale graduated in single degrees, with finer interpolation possible on premium instruments.
• Index gear: A circular disc, mounted concentrically with the quill, bearing a fixed number of equally spaced notches — commonly 96, though 64, 80, and 120-tooth gears are also standard. A spring-loaded pawl drops into each notch, locking the quill at precise rotational increments. The index gear is the mechanism that ensures each facet in a given tier is placed at an identical angular interval from its neighbours, producing the symmetry that defines a well-cut stone.
• Height stop: An adjustable mechanical stop that limits how far the quill descends toward the lap, allowing the cutter to repeat a facet to the same depth across multiple index positions without re-measuring.

Operating Principle

The cutter works from a faceting diagram — a schematic that specifies, for each facet or group of facets, the lap angle (in degrees from horizontal), the index positions, and the sequence of cuts. The operator sets the angle on the cutting head, drops the pawl into the appropriate index notch, and lowers the stone onto the rotating lap, applying light, controlled pressure. When the facet reaches the correct size — judged visually, or by reference to the height stop — the stone is lifted, the index is advanced to the next position, and the process repeats. After all facets in a tier are cut at a given angle, the lap is changed and the sequence is repeated for polishing.

The process is divided into two broad stages: cutting, in which coarser diamond laps remove material and establish the facet planes, and polishing, in which progressively finer surfaces refine those planes to optical clarity. The transition between the two requires careful attention to meet points — the junctions where adjacent facets converge — which must align precisely if the finished stone is to display the intended optical performance.

Principal Manufacturers

The modern faceting machine market is dominated by a small number of established manufacturers, each with a distinct design philosophy.

• Ultra Tec (Santa Ana, California) produces machines widely regarded as the benchmark for precision, featuring a micrometer-controlled quill and extremely tight mechanical tolerances. Ultra Tec instruments are the choice of many competition faceters and professional cutters working in high-value material.
• Facetron (Facetron Company, USA) offers a design notable for its integrated angle and index system and has a long following among serious hobbyists.
• Graves (Graves Company, Florida) manufactures machines at a range of price points and has historically been a common entry point for new faceters.

Beyond these American makers, machines are produced in Australia, Germany, and Thailand, and a substantial secondary market exists for vintage instruments. The choice of machine is consequential: backlash in the angle mechanism, runout in the lap spindle, and flex in the mast all translate directly into inconsistency across facet planes.

Accuracy and Limitations

Even a well-maintained machine introduces small systematic errors. Lap runout — deviation of the lap surface from a true plane — is perhaps the most significant, as it causes the effective cutting angle to vary slightly across the lap's radius. Experienced faceters compensate by cutting at a consistent radial position on the lap and by periodically truing the lap surface. Mechanical backlash in the angle adjustment requires the operator always to approach the desired angle from the same direction. Temperature changes can cause differential expansion in the machine's components, a consideration of real importance when cutting to competition tolerances.

These limitations notwithstanding, a competent operator working on a quality machine can routinely hold facet-angle accuracy to within a fraction of a degree — sufficient to produce stones whose optical performance is indistinguishable from the theoretical ideal of the design being cut.

Further Reading

• Gems & Gemology — GIA
• International Gem Society: Faceting Machines