Part 3

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.

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 underside of the cutter’s wrist, it should be permitted to cool for a few seconds before continuing. After polishing on the buffing wheel, the gemstone should have a fine, high polish and be free of any scratches or surface irregularities. The finished gemstone is removed from the dop-stick by heating the dop-wax and pulling the stone loose. Any excess wax that hardens again before it can be removed from the stone by hand can be dissolved away by rubbing with an acetone-soaked cloth. Figure 5 illustrates the desired appearance of the gemstone at the end of each of the steps of cutting and polishing.

Faceted Gems

The 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.

TOP VIEW SIDE VIEW Star facet Crown main facet Crown girdle facet Pavilion girdle facet Pavilion main facet TABLE CROWN GIRDLE PAVILION CULET BOTTOM VIEW

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.

Water Light Adjusting ring Post Arm Chuck Stone Abrasives DIAMOND DUST CALCIUM OXIDE LANDE-A

CHUCK DOP-STICK DOP-WAX STONE LAP

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.

TABLE FACET DOP-WAX STONE DOP-STICK

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 the preforming process is completed, the stone should have the desired outline of the finished gem (fig. 10).

DOP-WAX STONE DOP-STICK

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.

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 are shorter. The crown girdle facets should be joined in exactly the same way as the pavilion girdle facets. When these facets are properly cut, the cutting angle is again changed, the arm rotated, and the eight star facets are cut. This completes the cutting of the crown of the stone. The cutting lap is removed from the lap plate, and the polishing lap is secured into place. The facets are carefully polished in the same order that they were cut. After the last star facet has been polished, the stone is removed from the dop-stick. Any excess dop-wax is removed from the stone by means of a solvent, and the full beauty of the finished gem is revealed.

Tumbled Gems

One 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.

Stone Dop-wax Dop-stick Chuck

TEXAS GEMSTONES

Amber

Composition: fossil resin. Crystal system: amorphous. Hardness: about 2.0 to 2.5. Specific gravity: variable, from 1.05 to 1.10. Luster: resinous. Color: brown, yellow, red, orange, and white. Streak: white to yellowish to gray. Cleavage: none. Fracture: conchoidal. Tenacity: brittle. Diaphaneity: transparent to translucent. Refractive index: variable, about 1.54. Burns with a sweet, piney odor.

Rich brown to yellowish amber has been found near Eagle Pass, Maverick County, in Cretaceous coal and on Terlingua Creek, Brewster County. Although much of this material is translucent and the quality suitable for lapidary purposes, the pieces are seldom more than a fraction of an inch in diameter.

Occasional finds of poor quality brownish amber have been reported from the Tertiary formations of the Gulf Coastal Plain, but thus far no gem quality material has been found.

The softness of amber limits its use to brooches, necklaces, and other jewelry that is relatively safe from abrasion.

Augite

Composition: CaMgSi₂O₄; may also contain iron, aluminum, and sometimes titanium. Crystal system: monoclinic. Hardness: 5 to 6. Specific gravity: 3.2 to 3.6. Luster: vitreous to dull. Color: dark greenish brown and greenish black. Streak: light grayish green. Cleavage: two directions, poor. Fracture: conchoidal to uneven. Tenacity: brittle. Diaphaneity: opaque to translucent. Refractive index: variable, about 1.60 to 1.71.

Augite of gem quality occurs near Eagle Flat, Hudspeth County, Texas. Although this material is very dark greenish brown and not commonly thought of as a gemstone, lapidaries have used it to fashion black faceted stones and cabochons that resemble obsidian. Most of the augite occurs as loose pieces and crystal fragments that have weathered out of nearby igneous rocks; the augite can also be found in situ in the igneous rocks.