SELECTION OF CARVING ROUGH
While it may seem that more latitude can be taken in respect to internal defects when choosing large rough for carvings, this is really not the case. As a matter of fact, it is far more difficult for the carver to obtain satisfactory rough than the cutter of gems. Because some types of carv¬ings require sections to be quite thin, such as the arms or legs of a statuette, even the slenderest of cracks cannot be tolerated unless the carver is able to adjust his design to place such defects where they will not be conspicuous. The Chinese carvers are past masters in disguising cracks and other defects, but this is facilitated by the complexity of their carvings which makes it easy to change the design slightly to accommodate any defect which happens to appear during carving. On the other hand, the traditions of Western art generally call for more simplicity, which implies the freer use of broad, smooth areas. This, of course, allows much less opportunity to alter the design if a defect appears. Cracks, patchy color areas, soft spots, and other defects are particularly unde¬sirable whenever large smooth areas are to be part of the carvings. These remarks apply equally to material which is to be used in bowls, large ash trays, book ends, and other massive objects.

IMPREGNATION OF ROUGH TO DISGUISE DEFECTS
Knowing that cracks detract from the value of rough, some overseas dealers in large parcels of rough, notably Brazilian and Indian dealers, sometimes practice a deception which the buyer should guard against. Continue Reading »

QUARRIED MATERIAL
Much cabochon rough is extracted from its host rock by the use of considerable physical force, perhaps even explosives. Sometimes the quarried blocks are so large that they cannot be easily transported and must be further broken down into manageable pieces by means of sledge hammers. If the material is valuable throughout, it may pay the operator of the mine to saw the larger masses into smaller blocks at the mine even if considerable difficulty is involved in doing so. Continue Reading »

MOONSTONES
The finest moonstones are feldspar gemstones occurring in Burma and Ceylon. Both countries provide rough remarkable for its high degree of translucency, almost water-clear in some specimens, and the presence of minute inclusions which give rise to a beautiful silvery or blue sheen. Any rough requires careful inspection before purchase because of the tendency of native dealers to attempt to dispose of unsuitable material.

Much of the rough is partly split along two prominent cleavage planes or sometimes into plate-like fragments which, at first glance, seem satis¬factory for cabochons. Unfortunately, such thin pieces often display the moonstone sheen only on the thin edges. This means that any gem cut from them would have to be so small that it would scarcely pay for the cost of cutting. If these fragments are cut in any other way, with the idea of obtaining larger gems, the sheen will be badly off center.

Much less troublesome is the feldspar moonstone which has recently appeared from India in the form of large blocky fragments. While good-sized gems may be cut from them, they seldom compare in translucency or strength of sheen to the fine moonstones of Burma and Ceylon.

SUPERIORITY OF WATERWORN OR WEATHERED ROUGH
Much high-quality material occurs in the form of pebbles and boulders which have eroded from the rocks originally enclosing them. Sometimes they are found near the place of origin, but at other times they are washed into streams and rivers and subjected to severe pounding before coming to rest in a gravel deposit. Only the soundest crystals and masses can survive this combined attack of weathering and stream wear, and it is for this reason that gravel beds rich in gems are the particular target of knowledgeable gem miners. It can be truly said that nature has tested each piece and destroyed those found wanting.

Stream-worn material is therefore likely to be of very high quality and includes the gem pebbles from Ceylon and Burma mentioned earlier, various durable gem species such as topaz, tourmaline, beryl, and chryso-beryl from alluvial deposits in Brazil and elsewhere, and waterworn agates such as those from the Montana river deposits. Nephrite jade and jadeite are famous for providing very fine material from pebbles and boulders. In fact, the older Chinese jade carvers vastly preferred alluvial material, either nephrite from the stream gravel deposits of Turkestan, or the jadeite boulders from Burma. These were viewed as having proved them¬selves tough enough to produce the most delicate carvings.

Weathering need not be connected with stream wear as is shown by the loose nodules of chalcedony found lying on the ground in many places in the world. The agate nodules and geodes of southern Brazil and northern Uruguay, vein sections of chrysoprase in Australia, petri¬fied wood in Arizona, and peridots and garnets in New Mexico and Arizona are some examples of gemstones weathered from their original deposits. Nature has destroyed the enclosing rocks in a gentle yet irre¬sistible fashion, leaving behind the more durable gem material. As far as freedom from defects is concerned rough of this kind can be of top grade because it has been treated gently during the long cycle of erosion. However, if cracks existed in the material from the time of its formation, they may still be present after it has lost its enclosing rock and, as a general rule, the quality of the material is not as high as that found in stream gravel deposits where a more severe elimination process has taken place.

SELECTION OF CHATOYANT ROUGH
The term chatoyant is used to designate the silky sheen caused by very small reflecting inclusions of a fibrous nature which lie in parallel posi¬tion in certain gemstones. Strictly speaking, all star gems are chatoyant, but the chatoyancy occurs in several directions at once while in ordinary chatoyant gems, such as catseye chrysoberyl and catseye tourmaline, only one set of reflecting inclusions is present and therefore only one line of light crosses over the polished surface of the gem.

The principles for selecting suitable chatoyant rough are the same as outlined above for star stones. Sometimes the selection task is easier because some chatoyant gems possess this property so strongly that even the briefest inspection is enough to convince one that the effect really exists. Good examples are tigereye from Africa, some chrysoberyls and tourmalines, and rarely, some beryls. Figure 14 shows how a catseye gem is cut from chatoyant rough and indicates the proper shape of the rough if a good yield is expected. Continue Reading »

CABOCHON MATERIAL
Flawlessness in cabochon material is not as crucial as it is in facet material; however, large flaws or surface cracks which tend to trap polishing powder, pores, soft inclusions which may pit during polishing, and other gross defects can result in unsightly gems. In very valuable materials such as star ruby and corundum, which depend for their value upon the inclusions creating the stars, great care is used in selecting the rough, because the brightness of the stars depends upon the basic clarity of the gemstone. If dark-colored inclusions are present, or small cracks and other defects, the reflections will be interfered with and the gem will be less beautiful. Some specific recommendations on star rough follow. Continue Reading »

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. Continue Reading »

COLOR AND COLOR DISTRIBUTION
In many gemstones, color tends to be uniformly distributed in the rough, and the lapidary’s only concern is that of cutting the largest gem consistent with the shape of the rough. In other gemstones, however, the color may be unevenly distributed or even changeable in character from place to place as shown in Figure 5. Amethyst is frequently an offender, with its color confined to narrow bands with colorless material between. If these bands appear edge-up as one looks down upon the gem, they will be easily noticeable and detract from the appearance. The skilled lapidary carefully notes their position and orients the rough in such a manner during shaping that the bands will lie approximately parallel to the top surface of the gem. Thus if one buys amethyst rough, it is neces¬sary to look for this defect and select only those pieces in which it will be possible to place the bands correctly and still obtain a good yield.
Sometimes color occurs in only one spot or area, the rest of the rough being pale in hue or even colorless. If the rough is shaped as shown in Figure 6, with the spot of color placed at the apex of the pavilion, the finished gem will be “flooded” with color and could appear quite hand¬some.

A particularly troublesome color problem arises in gemstones which show marked differences in color depending upon the direction within the crystal. When two colors appear, the effect is called dichroism and may be present in such gemstones as tourmaline, beryl, apatite, benitoite, ruby, sapphire, and zircon. The effect is often so strong in tourmaline crystals that it can be easily detected with the unaided eye. The dichroic effect is shown diagrammatically in Figure 7. Very commonly, the color along the length of the crystal, or the direction of the optic axis, is a disagreeable shade of olive which is sometimes so dark that it seems black. However, looking through the sides of the crystal, the color is a much more pleasing shade of yellow-green. Figure 7 shows how a step-cut gem must be placed in such a crystal if the final color is to be pleasing. In some dichroic gems the lapidary reverses the procedure to obtain the best “face-up” color. For example, the best color in a pink tourma¬line is seen looking down along the crystal axis instead of across; thus the gem has to be cut so that the top facets lie across the crystal instead of parallel to its length. Other dichroic gemstones that require this ori¬entation are ruby and sapphire and some beryls. To be certain of the proper orientation, some cutters use a small instrument called a dichroscope which separates the two colors clearly and enables the skilled user to tell which direction in the rough gemstone would produce the best results. One of the simplest of these instruments is made from pieces of polaroid film; its use is shown in the photograph actual crystals of pink tourmaline are being tested. The differences in color shade are apparent in the two windows of the instrument.
In still other kinds of gemstones, three colors occur in the crystals according to direction, and the effect then is given the name trichroism. It is more difficult to orient a gem properly in such crystals, but again the dichroscope is helpful. The cutter examines such crystals with care to observe the separate colors. Only two colors can be seen at one time, but by turning the crystal about he eventually detects the third color. He then makes a choice of direction based upon the most pleasing and strongest color, remembering that now at least two of the three colors will blend into a combined hue. Some experience is necessary in the cutting of such crystals, but once the tricks have been learned it is not a difficult procedure. Striking trichroic colors are seen in iolite (pale yellow, pale blue, deep blue) and in facet-grade andalusite (yellow, green, red).
It now only remains to say that in a third class of gemstones there is no color difference according to direction and of course no inspection of the rough is required with the dichroscope, the lapidary needing merely to look for possible patchiness or zoning of color. Some gemstones in this class are garnet, opal, natural glasses (tektites), obsidian, and spinel. Those gemstones that would normally show dichroism or tri-chroism but are composed of a great many minute interlocked crystals also behave in this fashion, e.g., jadeite and serpentine. If highly translucent specimens of these gemstones are held before the dichroscope, no color changes will be detected.
The table on page 39 shows the principal gemstones exhibiting pro¬nounced dichroism or trichroism. Strong color differences require proper orientation if satisfactory gems are to result.

EFFECTS OF FLAWS AND INCLUSIONS
Ordinary cracks and fissures are abundant in many gemstones but may also arise during trimming of the rough by miners and dealers. When¬ever possible, cracked rough should be avoided for the reasons explained below. If the edge of a crack intrudes into a clean area, one may be sure :hat the base of the crack extends deeper than the eye can see, because its narrowest part is thinner than one wave length of light and thus becomes invisible. Despite its invisibility, the damage exists and it may happen that during cutting the crack will open farther. Continue Reading »

The following sections discuss important properties and characteristics of rough gemstones which the buyer must take into consideration when making his purchases. It is not within the scope of this catalog to ex¬plain the causes of these properties and characteristics, or to describe the operation of the several gemological instruments which will be men¬tioned from time to time. The reader desiring more information should consult one of the gemological textbooks listed in the Bibliography. The last part of this chapter provides a complete and detailed listing of the gemstones which have been used by both commercial and amateur lapi¬daries. Each entry gives pertinent remarks designed to assist the reader in becoming a discriminating buyer.

FACET-GRADE MATERIALS
Obviously, the first consideration in choosing material for faceted gems is that it be transparent enough to permit the development of the bril¬liance for which faceted gems are mainly valued. Cloudy to translucent gemstones, and even those which are quite opaque, are sometimes cut into faceted gems, but they cannot be as spectacularly beautiful as those which are perfectly clear inside. Occasionally, highly translucent chryso-colla chalcedony, smithsonite, idocrase, serpentine, and other normally massive materials are facet-cut and show enough diffuse reflection from the back facets to make them glow in a rather pleasing manner.
Ideally, however, facet material should be completely “clean,” or free of flaws of any kind such as cracks, partly developed cleavages, or solid inclusions or cavities containing liquids or gases. Common inclusions consist of layers of minute gas- or liquid-filled cavities, the shape of the layers suggesting the name “veil” or “veil-type” inclusions. Other common inclusions are hollow openings or solid inclusions, of small diame¬ter and great length, often resembling bundles of hairs in parallel posi¬tion. While inclusions of this kind serve useful purposes in cabochon gems, they are generally regarded as defects in facet-grade material.