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.