  There are two types of widely used gemstone cuts. Opaque or figured gemstones are usually cut with a rounded upper surface and a flat or rounded back. A stone cut in this fashion is termed a cabochon or is said to be cabochon cut. There are several variations of this mode of cutting (fig. 2). Precious opal, agate, jade, star sapphire, and fossil wood are some of the stones that are cut mostly as cabochons. Transparent gemstones are usually cut with many plane polished surfaces. Such stones are called faceted, and the process of cutting and polishing these stones is called faceting. Emerald, diamond, topaz, and garnet are examples of gemstones that are commonly seen as faceted stones. Fig. 2. Variations of the cabochon cut. Left to right: double cabochon; flat cabochon; simple cabochon; hollow cabochon. The cutting of gemstones, although sometimes tedious and time consuming, is not especially difficult or complex. However, like most arts and crafts, technique and ability should improve with practice and experience. There are currently many amateur gem cutters in Texas. A complete set of equipment necessary to cut cabochon stones may be purchased for as little as $50.00 or $60.00. Most amateur cabochon cutters have equipment that cost less than $100.00 which enables them to do very fine work on many gem materials. Facet cutting requires more precise equipment, and a complete array of such usually costs more than $100.00, although less expensive equipment can be obtained. The beginning gem cutter or lapidary who is willing to assemble and make some of his own equipment can reduce his initial expenses considerably. Cabochon GemsThe procedures listed herein for gem cutting do not apply to all gemstones. Stones that are especially brittle, soft, or difficult to polish require additional procedures or special techniques. Many lapidaries may deviate from these procedures. Some of the steps of cutting and polishing are merely matters of personal opinion and vary somewhat from cutter to cutter. There are several detailed texts on the art of gem cutting; the descriptions herein are designed to give the reader only a general idea of the procedures and techniques involved. The cutting and polishing of cabochons require several steps. The initial step is sawing. Assuming that the rough gem material is large enough to be sawed (larger than about half an inch in diameter), it is clamped into the carriage of a diamond saw (fig. 3) and cut into slices about ?-inch thick. The blade of the saw is mild steel that has been impregnated with diamond dust around the edge, hence the name diamond saw. The blade is rotated rapidly, and the material to be cut is “fed” to the blade by a sliding carriage on which the gem material is clamped. The extreme hardness of the diamond dust in the edge of the blade enables the saw to cut through several inches of gem material in a few minutes. The lower portion of the saw blade is immersed in a mixture of kerosene and oil, and the rotating saw blade carries with it some of the kerosene-oil mixture; this acts as a coolant and lubricant for both the saw blade and the material being cut. Without this lubricant, the heat generated by sawing would shatter most gem materials and also damage the saw blade. As this “slicing” or sawing of the material usually takes several minutes, a weight Fig. 3. Diamond saw.
After being sawed, the slab of gem material is examined, and the location and size of the stones to be cut from the slab are determined. The desired outline of the shape of the gem to be cut is marked on the slab with a pointed piece of aluminum rod; ordinary pencil marks are not used because they wear away too quickly in the cutting process. Once the area from which the gem is to be cut has been selected and the outline of the gemstone has been marked on the slab, the excess material is trimmed away by a smaller diamond saw known as a trim-saw. In some slabs the excess material can be broken and “nibbled” away with a strong pair of pliers. The remaining portion of the stone is usually held by hand and ground to the desired shape using the previously scribed mark as a guide. This is done using a relatively coarse-grained (about 150 grit) specially made carborundum grinding wheel. Now that the desired outline has been obtained, the stone is firmly affixed to a slender wooden or hollow aluminum dop-stick Fig. 4. Cabochon properly attached to dop-stick.
The top of the dopped gemstone is worked against the coarse carborundum grinding wheel until it is a rough approximation of the desired shape. The stone is then worked against a much finer-grained (about 220 grit) grinding wheel to remove the irregularities left by the coarse grinding and to further smooth and shape the surface of the gemstone. At all times while grinding, a small flow of water should be directed on the grinding wheel to keep the stone cool. Grinding on the stone for even a few minutes without cooling may result in the shattering of the gemstone because of heat created by friction of the stone against the grinding wheel. If the lapidary keeps the surface of the grinding wheel wet, there is little chance of damaging most gem materials. The next phase of cabochon cutting and polishing is sanding. The gemstone is worked against two sanding drums of different grit size. This sanding can be done with the sandpaper surface either wet or dry, as needed or as preferred by the lapidary. However, great care should be exercised during sanding so that the stone is not overheated. Overheating can easily occur whether the sandpaper is used wet or dry. As in grinding, sanding is first done on coarser grit paper (about 300 grit) and last on finer paper (about 600 grit). It is in the sanding process that the first hint of polish is noted on the surface of the stone. After sanding, the gemstone should have perfect form with no surface irregularities, a very finely textured surface, and only very minor scratches left from sanding. The gemstone is now ready to be polished. Fig. 5. Cabochons at various stages of cutting and polishing. Left to right: trimmed from slab: ground to outline; after rough grinding; after sanding; polished. At this point the procedure depends on the nature of the gemstone being polished. Most gem materials are worked against a buffing wheel that is impregnated or saturated with a mixture of some polishing compound and water. A soft felt buffing wheel with cerium oxide as the polishing agent is used for many materials. The mixture of cerium oxide and water is usually applied to the buffing wheel with a small brush. The lapidary should once more be careful not to overheat the stone. If the stone becomes too hot to hold to the Faceted GemsThe principles involved in faceting are about the same as those in the cutting of cabochons, but the equipment and technique are considerably different. The equipment required for the facet cutting of gemstones is built into or attached to a small specially constructed table (fig. 7), and the unit is commonly called a facet table. Most faceted gemstones are cut to obtain the largest flawless stone possible from the rough material. Therefore, one of the first and most important steps for the lapidary is to decide how the stone is to be cut from the rough crystal or pebble. The colors that can be obtained from the gemstone must also be considered, and the cutting of the stone oriented so that its best color is displayed. The lapidary also selects the orientation of the stone in relation to the cleavage or cleavages. It is difficult or impossible to polish facets of gemstones that are cut parallel to a good cleavage direction. Fig. 6. Nomenclature of the standard American brilliant cut.
Once the orientation of the gemstone to be cut from the rough material has been determined, the stone is dopped onto a special metal dop-stick that fits into the chuck of the facet head. The chuck is tightened so that the position of the stone on the end of the arm of the facet head is firmly fixed, and the facet head is adjusted so that the first facet that is cut is the horizontal, top facet of the stone or table facet (fig. 6). The table facet is cut by grinding the gemstone on a flat cutting lap that is diamond impregnated (fig. 8). By minor adjustments of the facet head, the lapidary can precisely control the location of the table facet. As soon as the table facet has been ground to the proper size, the cutting lap is removed from the lap plate, and the polishing lap is secured in place. Many different kinds of polishing laps and polishing compounds may be used depending on the properties of the material being polished. However, one lap and one polishing compound are usually sufficient for each gem variety. After the polishing lap is secured to the lap plate, the lapidary adjusts the facet head so that the stone is in exactly the same position relative to the lap that it was during the cutting of the table facet. The polishing lap is run wet or damp with water, as is the cutting lap, and small amounts of the polishing compound are applied to the surface of the lap while the facet is being polished. The minor scratches left by the cutting process are gradually removed, and a fine lustrous polish develops on the facet. It is especially important to take care in achieving a perfect polish on the table facet, as this facet occupies a large area of the crown of the gemstone. When the cutting and polishing of the table facet are completed, the gemstone is still rough or uncut in all portions except for this single, large, polished surface. Fig. 7. Facet table.
Fig. 8. Grinding the table facet on a rough stone.
The gemstone is then removed from the dop-stick by melting the dop-wax and is dopped once more so that the plane of the polished table facet is perpendicular to the axis of the chuck and arm of the facet head (fig. 9). Great care should be taken by the lapidary to insure that the table of the stone is exactly perpendicular to this axis, or the proper placing of the later facets on the stone may become very difficult. Fig. 9. Stone dopped to table facet.
Once the stone has been properly dopped to the table facet, the lapidary is ready to proceed with the cutting of the outline of the stone. If it is to be a brilliant cut, the stone is ground perfectly round in outline; if it is to be an emerald or step cut, it is shaped so that it is square or rectangular in outline. This process is called preforming. The arm of the facet head is lowered on the post until it is horizontal, and the stone is worked against the cutting lap until the desired shape is obtained. When Fig. 10. Preformed stone dopped to table facet.
The lapidary is now ready to proceed with the cutting of the pavilion of the stone. The arm of the facet head is raised to the proper angle for cutting the main pavilion facets. The angle at which the main facets are cut is very critical in determining the beauty of the finished stone. The required angle at which these facets must be cut varies with the refractive indices of the different varieties of gem minerals. If the facets are not cut at exactly the proper angle, light entering the top or crown of the gemstone can pass completely through the stone, instead of being reflected back out of the crown facets. The result is a dull, lifeless stone that appears to have a “hole” or “fish-eye” in the center. Stones that are cut in this manner are greatly reduced in value. The angle at which the facets are cut is controlled by the adjustment of the height of the arm of the facet head on the post. The lapidary will continually adjust this height, because the angle between the arm and the surface of the lap changes slightly as the facet is ground down to its proper place and size. Fig. 11. Proper sequence of cutting of the pavilion facets. Left to right: four main facets; all eight main facets; half of the pavilion girdle facets; completed pavilion. The standard American brilliant cut will be used as an example of facet cutting. Procedure for all other cuts is essentially the same to this point. After the eight main pavilion facets have been cut, the cutting angle is changed a few degrees, the arm of the facet head rotated slightly, and the sixteen pavilion girdle facets or “skill” facets, as they are often called, are cut (fig. 11). The pavilion girdle facets should meet exactly in the center of the main facets at the girdle of the stone. The pavilion girdle facets should neither overlap, nor should there be any space between them (fig. 12). After the pavilion girdle facets are cut, the cutting of the pavilion of the gemstone is completed. The facets are then polished on the polishing lap at the same angles and in the same order as they were cut, and the pavilion of the gem is completely finished. The stone is then removed from the dop-stick by melting the dop-wax and is re-dopped to the pavilion facets so that the crown of the stone is now exposed for cutting. Before the lapidary proceeds with the cutting of the crown, it is necessary that the stone be perfectly centered on the dop-stick and that the plane of the table facet be perpendicular to the dop-stick and to the axis of the arm of the facet head. The eight main facets are cut first, with numerous adjustments being made by the lapidary to insure that the proper angle is maintained (fig. 13). Then the cutting angle is changed a few degrees, the arm of the facet head rotated slightly, and the crown girdle facets are cut. The crown girdle facets are placed very similarly to the pavilion girdle facets except that they Tumbled GemsOne other method of finishing gemstones that deserves mention is tumbling. “Baroque” or “free-form” stones are produced in this manner. Loose pebbles or pieces of gem materials left over from other cutting processes are placed in a small barrel or specially constructed box with loose carborundum grit. The barrel is turned by means of a small motor, and the abrasion of the pebbles and grit against each other tends to round the pebbles and give them a finely pitted surface. Progressively finer and finer carborundum grit is used, and eventually a polishing compound. The result is several pounds of well-polished gem pebbles of various shapes and sizes. These baroque stones have found recent favor in costume jewelry of modern design. The tumbling process is rather slow, commonly requiring several days or weeks. However, little effort is involved on the part of the lapidary, and, consequently, the cost of most tumbled or baroque stones is quite modest. Only gem material that is unsuitable for cutting in other manners should be finished in this way. Fig. 12. Proper placing of the pavilion girdle facets. Left: facets not joined. Center: facets overlapped, joined too high. Right: correct placing.
Fig. 13. Proper sequence of cutting of the crown facets. Left to right: four main facets; all eight main facets; half of the crown girdle facets; completed crown. |
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