Gem Cutting Technology: Lapidary Equipment & Techniques
Cutting a Rough Gem Stone
Cutting a raw stone into a faceted and polished gemstone is a multi-step process. Each step is critical to the final outcome. The steps are:
- Girdling (Bruting)
- Faceting (Cross Working)
Marking: A rough stone is marked prior to cleaving or sawing to determine the direction of the grain or "cleavage", eliminate waste, and bypass inclusions or imperfections. The natural shape of the rough stone will also be a major factor in deciding how to cut the stone.
Cleaving: The term "cleaving" refers to splitting a stone along its grain by striking it. A rough stone is cleaved if there are conspicuous defects and/or inclusions which would prevent it from being made into a single gemstone.
Sawing: The rough stone is cut to a shape that approximates the shape of the finished cut stone but without the facets.
Girdling: The rough is placed in a chuck on a lathe. While the rough stone rotates on the lathe, a second diamond mounted on a dop is pressed against it, rounding the rough diamond into a conical shape. This step is also referred to as rounding or bruting.
Faceting: The cutting and polishing of each facet is accomplished by attaching the stone to a dop stick and pressing it against a revolving lap (see "Facetron" below). During this faceting stage the angles of each facet must be cut in order to maintain symmetry and produce maximum brilliance.
Brillianteering: In diamond cutting, if the primary faceting or "cross-working" is done by a separate craftsman, the final 40 facets of a round brilliant cut diamond's 58 facets will be cut by a Brillianteer.
Gemstone Color Evaluation
When cutting certain types of gemstones that have peculiarities with their color such as zoning (sapphire, tanzanite, etc.) or color shifts (ametrine, tourmaline, etc.), it is critical to determine the best orientation of the cut within the rough gem.
For evaluating the color of a gemstone, it is crucial that the identical light source, intensity, and color temperature is used every time. A "Gem Light Box" gives off a stable 4800 to 5200 Kelvin degrees (natural, indirect sunlight). Some light boxes have a UV component to the light for grading pearls or colored stones (Fraunhofer Solar Spectrum Chart, below).
Faceting Design & Rough Evaluation
Diamond Manufacturing factories use sophisticated electronic equipment for cutting and evaluating cut diamonds. Using the latest hardware and software to create highly accurate 3D models, these scanners measure the angle of inclination of a facet and its azimuth, allowing the operator to pre-visualize a 3D model of the cut stone. HeliumPolish Scanners are used for Round Brilliant Cuts as well as Fancy cuts. A device called a Pacor Oxygen Scanner can be used for optimizing rough stones based on purity to evaluate inclusion removal or reorientation.
Girdling or bruting the rough is a critical phase where the stone is placed on a lathe while a second diamond mounted on a dop is pressed against it, rounding the rough diamond into a conical shape. AutoBruters use the latest technology to preform "non-contact" measuring to overcome inherent problems in the rounding process. using live video and image analyzing software, potential out-of-roundness and any deviations from the desired dimensions and a computer will guide the machine to take necessary actions by slowing the bruting speed or pressure.
An "IdealScope" or "H&A Viewer" (below, center) uses a 10x lens with a pink/red reflector positioned in front of the diamond under a central viewing hole, allowing the viewer to see how much of the red/pink light refracts back from the diamond. The resulting pattern will be a good indicator of faceting proportion and symmetry.
For performing Cut Analysis on a finished stone, a Dia-Analyser (above, right) uses a camera to take photographs of the finished diamond while it is being rotated. The computer's software will digitize and the analyze the data to quantify all of the cut parameters.
Lapidary & Gem Cutting Equipment
The machine to the upper left (Poly-Metric Scintillator 88 Digital) and to the upper right (Facetron) are semi-automated faceting machines. Machines such as these have taken some the guess-work out of stone cutting, but a skilled craftsman must cut a rough stone to its optimal size and and take ito account any inclusions or imperfections that must be eliminated in the cutting process.
By examining the rough stone under a microscope or jeweler's loupe, the gem-cutter will decide which type of cut will show the stone's best color attributes, and what (if any) inclusions to avoid.
Facetron & Lapidary Gem Saw
The Facitron (left) and Scintillator (upper right) are water-cooled faceting machines designed to make cuts at precise angles by mathematically plotting out to depth and degree of a given facet. The rough gemstone is held by a chuck called a "dop" "dop stick" or "dop chuck" and ground against a grinding plate called a "lap". The rough stone is held to the dop with hot-wax glue.
By adjusting a dial indicator (protractor) the gem-cutter can control the degree of cutting angle to a tolerance measured in hundredths of an inch. The image above (bottom, center) shows a rough gemstone attached to a dop stick. The image to the right (above) is a combination lapidary gemstone saw (slab saw) and grinder.
If the material to be cut and polished is translucent to totally opaque (agate, onyx, malachite, etc.) a cabochon cut will most likely be employed.
Gem-Cutting and Lapidary Info
PrettyRock, Step-by-Step Gem Cutting . www.prettyrock.com
Gemstone Artists, The Gem Cutting Process . www.gemstoneartist.com
Faceting Machines, Faceting How-To Tips . www.facetingmachines.com
Rock Hounds, Faceting By Hand . www.rockhounds.com
Gem Society, Fundamentals of Lapidary Faceting . www.gemsociety.org
US Faceters Guild, Faceting Diagrams & Gemstone Designs . www.usfacetersguild.org
Bowers Museum, The Art and Nature of Precious Stones . www.bowers.org
Folds, Reflection & Refraction of Light in a Diamond . www.folds.net
AGL, Measuring Color Via Spectrophotometer . www.gis.net
Diamond Cut, HCA IdealScope . www.diamond-cut.com.au
Folds, Diamond Design - by Marcel Tolkowsky . www.folds.net