cutting diamond
Gemological Institute of America

GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber, Geologist & Gemologist
Emporia State University
Emporia, Kansas USA

http://academic.emporia.edu/abersusa/go340/cut.htm

Gem Fashioning


Mr. Jack Greer, a lapidary from Emporia, KS,
giving a demonstration on faceting gems.

Photo date 10/99; © by S.W. Aber

Introduction Cabochon Cuts Faceted Cuts
Tumbling and Inlay Cutting Cabochons Faceting Gems
Jack Greer Fashioning Workshop and Equipment

Introduction

Fashioning gemstones is accomplished by a diamond cutter or lapidary who carves or engraves, tumbles, cuts en cabochon or facets gem materials. Carved gems have the entire gem cut into a design, while engraved gems simply have a design cut into the gem. Carving and engraving are methods with a long history and include cameos, intaglios, and scarabs. Tumbling produces a polished, although irregular-shaped or baroque bead. The cabochon cut produces a curved or convex, smooth surface, while faceting creates a highly symmetrical shape, with flat, planar surfaces.

Early gem cutting and fashioning usually worked with the shape of the rough material, that is a stream worn pebble would simply be polished. Carved jade has been found with Chinese burial sites dating from 3000 BC (Hurlbut and Kammerling, 1991, p. 181). Cylinder seals were used as signatures and beetle-shaped scarabs were common in ancient Egypt. Advanced machinery and techniques would eventually give rise to improved cabochon cuts and faceting, which is more dependent upon the physical and optical properties of gem rather than the shape.


Photo date 10/99 © by S.W. Aber
The purpose of cutting a gemstone is to maximize the beauty. In general, transparent gemstones are faceted to enhance interaction with light, thus improving the brilliance (amount of light reflected), scintillation (sparkle and flashes), and fire (spectral colors). This light interaction enhancement is through reflection, refraction, and selective absorption.

Photo date 10/99;
© by S.W. Aber
Translucent or opaque gemstones are cut en cabochon or carved, maximizing the surface reflection of light and displaying unique color banding or patterns in the gem. A desirable optical effect can also determine the cutting style (e.g., asterism and the chatoyant cat's-eye require cabochon cuts).
Photo date 10/99;
© by S.W. Aber

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Cabochon Cuts


Photo date 10/99;
© by S.W. Aber
There are five basic styles of cabochon cuts: single, double, lentil, hollow, and reverse. The general outline, or shape as viewed from above, is circular, oval, rectangular, heart-shaped, or irregular. The single cabochon has a convex or arched up top and flat base. The double cabochon has both a convex or arched top and bottom, with the top usually higher than the bottom. The lentil cabochon is a double cabochon with both the top and bottom curvature equal, rather thin and flat. The hollow cabochon has a convex top and flat bottom, but the dome is hollowed out and polished. This concave surface enables better light transmission in dark stones. The reverse cabochon, although rarely seen, has the convex top with a concave or depression in it and flat bottom.

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Cutting Cabs

Cabochons are fashioned using sawing the rough, grinding, sanding, and polishing. Rough gem material is first reduced in size using a saw, with a diamond or carborundum impregnated saw blade. The blade runs vertically and is cooled by water or oil. The blade is very thin and made of copper, bronze, or steel. The specimen is hand fed into the saw or clamped into a vise and fed mechanically. A slab of the gem material is cut to the desired thickness. After sawing the gem, grinding, sanding, and polishing will finish the operation.

Photo date 10/99; © by Lesley
Zoch, reproduced with permission.


Photo date 10/99; © by S.W. Aber
The thin slab can then be smoothed of saw marks with a vibrating lap. Lapping is a simple process that consists of rubbing the stone against the metal lap or simply laying the stone inside the rings, as shown to the left. The rings keep the specimen from vibrating off the edge of the lap. The metal lap or flat plate is charged with loose abrasive. When water is added to the center of the moving plate, the resulting slurry grinds and polishes the sawn specimens to a flat surface. These slabs could be finished as book-end sections or readying the specimen for cabbing.
The slab is smoothed to whatever degree is desired. Finer and finer abrasive grits are applied to the lap surface. Grit is powdered natural or synthetic minerals of differing hardness used as an abrasive to smooth and polish gem materials. A no. 200 grit and water may be the first grinding, followed by a no. 600 grit. After the second grinding the surface has a frosted finish it is ready to polish. A cloth-covered pan with tin oxide or other polishing agents will smooth and finish a surface. The specimen should be dipped in water often and the face wiped clear of grit to observe the progress.
Photo date 10/99; © by S.W. Aber.


Photo date 3/02;
© by S.W. Aber
The vibrating lap is fine for smooth, flat surfaces, but not for a curved or convex surface. If a cabochon cut is desired, the grinding wheel or sanding belt is needed. The grinding wheel can create the rough shape of the piece and the curved or convex surface. The wheel can be made of silicon carbide, some other synthetic material, or even sandstone. Coarse grit wheels are used for the basic shaping and finer grit wheels are used for more precise removal of material.
Photo date 10/99;
© by S.W. Aber
Jack Greer shapes the rough on a grinding or sanding lap. Bookends, coasters, sculpture, or any variety of uses beyond cabochons for jewelry purposes, may result. Grinding this way shapes the stone to a desired configuration on a wheel using no. 80-100 grit, followed by a second pass with no. 200 grit. Water is used as a coolant. Sanding is done to remove the scratches of grinding and prepare the stone for polishing. A no. 400 grit is used for coarse sanding, and no. 600 grit for finishing. This is accomplished on a disc, drum, or belt sander. Dry sanding may crack stones, so wet sanding is usually preferred. Polishing is the final step and done on a felt or leather buff (used for dopped stones), using tin oxide or cerium oxide mixed with water to a thick cream and applied to the buff with a brush.
Photo date 10/99; © by S.W. Aber

Photo date 10/99;
© by S.W. Aber
Therefore, to create the cabochon cut, the specimen is sawed to size, marked with a template, cut to this outline shape, grinded, sanded, and finally polished. Various templates are shown to the left, in both metal or plastic, and used to draw the outline or desired cabochon shape on the slab using a colored pencil or aluminium pencil. After crudely shaping with a trim saw, the blank is ready for grinding or shaping.
Photo date 3/02; © by S.W. Aber

Photo date 3/02;
© by S.W. Aber
Mr. Greer explains the method of holding the gem to be fashioned. The specimen will probably be hand held in the beginning, but later it is fastened to a dowel rod, called a dop stick.

The dop stick is the stone dopped or cemented to a wooden stick, pencil, or nail. The gem is held to the dop stick with a hot wax mixture, called doping wax, which is heated to the melting point. The stick is dipped in the hot wax and layered on to form a blob on the end. The stone is heated and dopstick reheated so the stone and wax are at approximately the same temperature when attached for a good bond.


Photo date 10/99; © by S.W. Aber

Photo date 10/99; © by S.W. Aber
Polishing and finishing can be a long process. While on the dop stick, the shaped, flat-topped piece is ground such that a steep bevel is around the edge. This is to widen the cabochon at the bottom so it will set securely in the setting. Several bevels may be needed to rough out the preliminary shape from the bottom to the top of the stone. The symmetry of the outline is very important. Now the curved surface can take shape, going over the entire stone using a firm pressure and constant movement.
Finishing work is done with finer grit and possibly a wet or dry sanding cloth on the wheel or rotating disk. Leather may be used to prepare the surface for polishing, although the wheel may be covered with felt, wood, or cloth. The gem is held against a rotating wheel or sometimes polished by hand, using a polishing agent. Metallic oxides are often the polishing agents, such iron oxide or rouge, chromium oxide, tin oxide, silicon oxide or tripoli, synthetic aluminum oxide or Linde A and B, and others.

After the stone is polished, it is removed from the dopstick by melting the wax over low heat and holding the stone with a cloth. Clean off any remaining wax with methylated spirits or by dipping it in ice water. The base may need flattening or polishing if working on a transparent or translucent stone. The finished stone is ready for mounting. The finished product can certainly be worth the wait! If you missed the lecture on ring mounting, visit the silver casting webpage. Also for more information on this style of cutting, see Cabochon making 101 cabbing with Tucson's Old pueblo lapidary club, http://www.rockhounds.com/rockshop/oplc_cab.html.


Designs by Ernie Herrick
Photo date 10/99; © by S.W. Aber

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Faceted Cuts


Photo date 10/99;
© by S.W. Aber
Faceting began in the fourteenth century, with the earliest fashioning simply to polish natural crystal faces (Hurlbut and Kammerling, 1991, p. 183). Faceting is almost exclusively applied to transparent gemstones because this type of cutting maximizes reflection and refraction, and therefore the gemstone's brilliance, scintillation, and fire. If the gem is correctly cut, then light that passes into the gem will strike the back facets, be reflected twice, and then return to the eye, that is if it hits at an angle that is greater than the critical angle. If the gem is not cut properly, the light entering the gem will be refracted out the bottom of the gem, or it "leaks" out from the stone, i.e. when light lands within the critical angle. The larger the gem's refractive index the smaller the critical angle; the smaller the critical angle, the shallower the stone can be fashioned and still maintain brillancy (Hurlbut and Kammerling, 1991, p. 183).
The faceted gemstones consist of three main parts, the crown, girdle, and pavilion. The crown is made up of a table (large facet at the top of the stone), upper main facets (8, kite shaped), star facets (8, triangular), and upper girdle facets (16, triangular). The girdle (also known as the setting edge) is the section between the top and bottom of the stone and defines the perimeter, thus the overall shape of the gem. The pavilion has lower girdle facets (16, triangular), lower main facets (8, kite shaped), and the culet (small facet where the pavilion facets meet at a point). The eight lower main facets actually create an octagon shape to the culet.

There are two basic faceted cuts, the brilliant and step. The emerald cut and round brilliant are the basis from which most other cuts are based. A mixed cut would consist of, for example, a brilliant cut crown and step cut pavilion. Although new cuts are introduced fairly often, in 1988 DeBeers introduced new cuts specifically designed for misshapen or colored diamonds, called the sunflower, dahlia, marigold, zinnia, and fire rose.


Jack Greer explaining the different facets.
Photo date 10/99; © by S.W. Aber

A cut developed prior to the sixteenth century was the rose cut or rosette. It has a faceted crown and no pavilion. This cut was popular for diamonds and garnets in Victorian jewelry (Hurlbut and Kammerling, 1991, p. 185). The table cut was another early cut which took advantage of the octahedral shape of many diamond rough, simply creating four facets on the crown with a table and four facets on the pavilion with a small culet.

The step cuts have rows of facets that resemble a stairway. The outline, or perimeter which is shown by the girdle, may be rectangular, square, triangular, kite shaped, among others. Long rectangular step-cut gems are known as baguettes, often used as side stones in a setting, while rectangular shaped and cut-corner or faceted corners is known as the emerald cut, often used with emeralds.

Brilliant cut gems have crown and pavilion facets, kite or triangular shaped, that radiate out from the center. The outline may be round, pear, oval, marquise, and more. The old mine and old European cuts were forerunners to the round brilliant. The cut was similar to the round brilliant, except for a squarish outline (old mine) and higher crown, smaller table, and large culet. The round brilliant cut is often used for diamonds, believed to be first utilized at the end of the seventeenth century (Hurlbut and Kammerling, 1991, p. 186). Cuts that vary from round though, have pavilion facets that do not line up at the proper angle, which results in light leaking out the bottom of the stone and a bow-tie effect. This darkened area in the shape of a bow tie reduces the brilliance.

  • For more information and images of faceted cuts visit

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    Faceting Gemstones


    Photo date 10/99;
    © by S.W. Aber
    Faceted gems are cut by holding a preformed stone, mounted on a dop stick, against a horizontal lap until a flat facet is ground. After a facet is formed, the dop stick is rotated and tilted, creating more facets in the overall desired shape. First rough faceting material is obtained. The image to the left is rough cubic zirconia.
    The gem material is mounted on a dop stick. This example shows the crown faceting nearly complete. Usually all the crown facets are ground and polished before working on the pavilion. After the crown or pavillion has been faceted, the gem is transferred to a second dop stick.

    Photo date 3/02;
    © by S.W. Aber
    After determining how best to use the rough material, a design is chosen. This supernova cut was taken from a book detailing angles needed for faceting.

    Photo date 10/99; © by S.W. Aber

    Photo date 3/02; © by S.W. Aber


    Jack Greer demonstrates the faceting using a faceting machine.
    Photo date 10/99; © by S.W. Aber

    Photo date 10/99; © by S.W. Aber
    Dripping water mixed with different powdered grits produce a slurry on the polishing wheel. The angle is chosen and the gem mounted on the dop stick is lowered onto the wheel. The lap has a mechanical head which holds the dop at the precise angular position determined on the scale.
    Once the crown is finished, the gem is transferred to another dop stick to cut the pavilion facets. The process begins again... Some finished gems, fashioned by Mr. Greer, are shown below.
    Photo date 10/99; © by S.W. Aber

    Photo date 10/99; © by S.W. Aber

    Photo date 10/99;
    © by S.W. Aber

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    Other Methods of Finishing Gems


    Photo date 3/02
    © S.W. Aber
    Cutting and polishing can produce a nice set of bookends. Carving and engraving is accomplished by holding the specimen up against engraving instruments or carving tools. The tools may be steel or silicon carbide burrs and disks similar to tools used by the dentist.
    Polishing irregularly shaped gem material can be done with tumbling. Tumbling is a method that mass produces baroque-shaped gems or material that is irregular in outline. Tumbled gems can be drilled and strung or cemented into settings. A tumbler is shown below.

    The barrel or drum of the tumbler rotates slowly and the process can take a week or more depending on the hardness of the specimens. The irregular pieces are placed inside the barrel, along with water and an abrasive. The continual abrading action wears off the rough edges; a finer abrasive is then added until finally the polish is complete.


    Photo date 10/99; © by S.W. Aber

    Photo date 10/99; © by S.W. Aber
    Another lapidary operation is inlay work. First a design is drawn and the outline created. Freshwater clams or oysters may provide the mother of pearl needed for the design.


    Photo date 3/02; © by S.W. Aber
    Each segment must be cut and fit into place.
    Photo date 3/02; © by S.W. Aber

    Photo date 10/99
    © by S.W. Aber
    Once the pieces are all cut, they are set into place using an epoxy cement.
    Photo date 3/02
    © by S.W. Aber
    Design by J. Greer

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    For more information and cutting equipment visit...



    The material for this section came primarily from:
    • Hurlbut, C. S., & Kammerling, R. C. (1991). Gemology. NY: John Wiley & Sons, Inc.
    • Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing.

    Return to the Syllabus or go on to the next lecture.

    This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: esu.abersusie@gmail.com Thanks for visiting! Webpage created: November 15, 2000; last update: 30 September 2012.

    Copyright 1999-2012 Susan Ward Aber. All rights reserved.