ORIENTING SYNTHETIC RUTILE
In view of the troubles many cutters have had with the highly double-refractive synthetic rutile, a few words on how to locate the direction along which the least blurring will occur in finished gems are considered worthwhile. This direction coincides with the optic axis. Synthetic rutile is usually sold in boules or cylindrical masses which look like enormous grains of corn, or sometimes in pieces of boules. The optic axis can lie in any direction in the boule, although mostly it is approximately par¬allel to the length. Unfortunately the outside of the boule is always frosted and one cannot look through it to detect the optic axis without polishing some facets or “windows” in the manner shown in Figure 11.
Near the end opposite these windows a pencil mark is made on the sur¬face at any convenient point. If by luck the pencil mark and the window through which the lapidary looks are located at opposite ends of the optic axis, then very little blurring of the pencil mark will be apparent; however, it is more likely that the mark will be appreciably “doubled.” Now comes the trick. If another mark is made about i/g” away from the first, and observed again through the window, it is easy to compare its appearance with that of the first mark. If it is more blurred, it means that the optic axis is being worked away from; if it is less blurred, it means that the optic axis is being approached. By making another mark, and perhaps others toward the proper direction, it is possible to steadily decrease the blurring image until the point is reached where blurring is least. Now if an imaginary line is connected from the window to the last pencil mark, this will be the direction of the optic axis. The boule can then be marked and sawed up properly, with confidence that when gems are cut the least amount of blurring will occur.
Except for the gemstones mentioned and a number of the rarely cut species, such as the carbonates, the effects of double refraction can be largely ignored when purchasing rough. In the case of synthetic rutile, however, it is extremely important, and every effort should be made to determine the optical axis before cutting begins. If a considerable num¬ber of synthetic gems are to be cut it is wisest to buy whole boules, be¬cause they lend themselves better to the axis-locating procedure described above than do sawed sections or the small angular pieces which are com¬monly sold for the cutting of small single gems. If small sections must be bought it is a good idea to ask the supplier to indicate with two opposed dots on the surface of the rough the position of the axis, which he should know if he has taken the trouble to saw the boule according to the axis position.
PRELIMINARY EXAMINATION OF FACET ROUGH
Perhaps the first question that the experienced lapidary asks himself when shown a parcel of facet rough concerns the shape of the gemstone crystals or fragments. If these are excessively thin, elongated, or grossly misshapen, the return in terms of finished carats against rough carats can be disappointingly low. Very slender pencils of tourmaline, for ex¬ample, regardless of how lively the color or how elegant the crystal form, cannot yield gems any thicker than the pencil itself (Figure 8). Thin platy crystals, Montana Yogo Gulch sapphires for example, are also im¬possible to cut into anything except very small gems. Similarly, crystal fragments with deep hollows must be cut to eliminate such hollows, which means a drastic reduction in finished size regardless of how large the piece of rough appeared at the start. In general, best returns come from chunky or blocky pieces approximating spheres or cubes, and thus lending themselves readily to the cutting of brilliant or step cuts. In any case, the cutter must visualize what the rough can yield and buy accord¬ingly, providing, of course, that the rough is satisfactory in terms of color, flaws, and the other considerations previously discussed.
Once the rough has been selected for shape, it should be critically examined for inner clarity, color and color distribution, cleavage traces, etc. Cracks or inclusions show up best in transmitted light, as shown in Figure 12 where a desk lamp is being used for examination. The stone is placed at the edge of the shade in such a way that the strong light masses through the gem but does not blind the eyes. By slowly turning the rough, brilliant reflections arise from cracks, and to a lesser extent, from inclusions. Sometimes it is helpful to wet the stone with oil or with water because this allows a clearer view of the interior.
As an excellent alternative, a small pencil flashlight can be used to examine rough by holding the stone near the body at a comfortable dis¬tance below the eyes and shining the light upwards through the stone. The best flashlight for this purpose is the kind fitted with a small bulb which is about the size of a match head. This puts a narrow but intense beam of light into the stone, which quickly illuminates blemishes.
IMMERSION OF ROUGH
A better way to examine the interior is to place a piece of rough in a liquid which nearly matches its refractive index. Everyone has observed the startling result when clear ice is placed in ordinary water; seemingly it disappears! Because ice is frozen water it has nearly the same refrac¬tive index as ordinary tap water; hence the “disappearance.” Precisely the same effect is observed with gemstones, providing the right fluid is used. The trick is to obtain a closely matching fluid, the closer the better. A fluid may be of any kind, as long as it is not harmful. If an exactly matching fluid cannot be obtained, practically any fluid is better than nothing at all. Some commonly available fluids are fuel oils, kero¬sene, mineral oil, castor oil, salad oils, and clove oil. The following table lists those which are most useful. Some of the unusual ones may be or¬dered from drugstores.
OTHER MEANS OF EXAMINATION
One of the best ways to examine facet rough is to cut and polish a small “window” on the rough to look inside. Not much material is lost in the process and uncertainties as to internal quality can be resolved.
Skilled cutters usually cut a single large facet on the rough where it will serve the double purpose of a window, and, if no defects are found, as the large table facet of the finished gem. It must be mentioned, how¬ever, that dealers do not permit gemstones to be returned to them if windows have been cut on the rough; buyers must make up their minds about the rough without damaging it in any way.
When a large quantity of rough is involved, especially if it is in the form of many small etched or abraded fragments or pebbles, individual examination becomes tedious and time-consuming. Sometimes it pays to make up an examination bench fitted with a frosted glass top, beneath which is an electric bulb. The pebbles are spread over the glass and the obviously clear ones separated from those which are just as obviously unsuited for facet work. Another method is to tumble the rough to a good polish, and from among the finished stones, select those of facet quality. This has been successfully employed with peridot rough from Arizona, rose quartz from Brazil, and other gemstones. While there is ;some loss of material, it is not as much as may be imagined, and the ability to select top-grade, high-priced material may more than offset the loss in weight and the cost of tumbling.