Cabochons: Tolerance, Dopping and Calibration
by Barbara van Look, Marketing Content Development Group, Exclusively for Fire Mountain Gems and Beads®

The shape "cabochon" (KAB-oh-shon) refers to a piece of gemstone (or other material) which has been shaped and polished, instead of faceted. Such stones have a domed front surface and a flat back.

How a Cabochon is Made

Cabochons are usually cut from a slice of gemstone rough or other materials using a calibrated stencil template, or in freeform shapes for customized manufacturing. Due to labor costs, many are now cut on a cam-cutter, a machine that completes 75% of the cutting process. Individual cabochons are then ground into shape with grinding disks or belts coated with silicon carbide or diamond dust. Cabochon cutting can be an expensive interest to get started with, even on a hobbyist level.

Cabochons (often shortened to "cabs") are cut using a handcrafting technique called dopping. Dopping involves adhering the cab to a dowel or rod with a hard wax, creating the curved or beveled edges, then sanding and polishing the top into a uniform dome. As most of this work is done by hand, the height and curve of the dome varies, which can affect how the cabochon fits in standardized settings.

Cabochons are "calibrated" to match popular or frequently used standardized setting sizes such as 20mm round or 40x30mm oval. Calibration refers only to this size-and-shape indicator. Cabochon qualities such as gemstone grades, polishes, finishes, beveled edges or carved surfaces are not affected by whether a cabochon is considered "calibrated" or "uncalibrated."

"Calibrated" cabochons can still vary greatly, even if they fit the measurement and shape guidelines. Cabochons are measured by their largest points: side to side and end to end. The arc or curve between those compass points can be shallow or deep, resulting in cabochons which are calibrated yet not identical in shape. Oval cabochons are the most variable, as they can vary between a narrow curve close to a marquise to a wide curve similar to a rounded rectangle.

Cabochons are rarely the same because of dopping, the range of Mohs hardnesses in cabochon materials making some stones difficult to cut and polish, and wear-and-tear on lapidary machinery. These factors contribute to varying calibrations from cabochon to cabochon.

Size Tolerances for Cabochons

Cabochons Fire Mountain Gems and Beads uses the following guidelines for size tolerances in cabochons, and pays extra to ensure that our cabs are as tight an allowance as is possible.

For cabochons which measure 6mm and smaller, the variation between cabs of the same size must be 0.10mm-0.20mm. Cabochons which measure more than 6mm are allowed a size tolerance of 0.20mm-0.30mm. Cabs made from bone, shell (including Abalone) and Coral can vary 0.50mm. Shell cameos, being a product of much hand-carving and finishing, can vary as much as 2mm smaller or larger than the listed millimeter size.

Using Cabochons

Cabochons can be used in calibrated settings, designer-created bezel settings and other artistic techniques.

Calibrated settings include Cab-Tite™, Wrap-Tite®, Bezellite, cinch mount and Almost Instant Jewelry® settings, as well as most solid-back and open-back bezel cups, prong-set and glue-in settings. Settings include pendants, drops, links, rings, bracelets and a variety of earring findings.

As the arc or curve of a cabochon's shape--as well as the height of its dome--can vary, jewelry designers may need to take extra care when selecting settings or mountings. Settings made of rigid metals such as brass and pewter may not fit all cabochons, even if both mounting and cabochon match in millimeter sizes. This is especially true of oval settings, as the curve of the oval in setting and cabochon may not be identical.
Calibrated Settings

Custom settings can be created for uncalibrated or freeform cabochons using bezel strips and metal sheet, or a range of metal clays: BRONZclay™, COPPRclay™, Art Clay® and PMC®.

Designer-artists can showcase non-calibrated cabochons in standard shapes--oval, round, marquise, teardrop and others--as well as freeform cabochons by adhering them to flat surfaces (such as Aanraku® bails), wire-wrapping around them, embedding in soft media such as Kato Polyclay™ and Apoxie® Sculpt or embroidering onto backing materials using seed beads.

The lapidary process of creating this shape is called cutting en cabochon, and is usually applied to softer gemstone materials, opaque gemstones and stones which display special effects like asterism, chatoyancy (sha-TOY-en-see) and iridescence (like star sapphire, tigereye and rainbow moonstone). The most common shape is an oval or ellipse.

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