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. This is likely to occur in kunzite, topaz, tourmaline, and clear opal, but it could hap¬pen in other gemstones. In the case of kunzite, such cracks have been known to open progressively during cutting, despite all attempts of the lapidary to cut past them, until finally the partly finished gem falls in two. The small cracks around the periphery of transparent tourmaline crystals and those in the clear “cherry” and “honey” opals of Mexico are also likely to continually expand during shaping, until the stone isground to a ridiculously small size or falls into pieces. For these rea¬sons, it is wise not to accept transparent rough in which well-developed cracks point toward clear interior areas.

Another drawback to leaving cracks in finished gems is that the slight surface openings may pick up polishing powder from the lap or create a series of small scores behind the crack opening which interrupt the smoothness of the polished facet. From the standpoint of brilliance and clean reflections, cracks are unsightly if allowed to remain near the cen¬ter of the gem, because they will be repeatedly reflected until the gem seems to contain dozens of cracks instead of merely one or two. This effect is most disastrous in step-cut gems and least noticeable in round brilliants, particularly if the cutter places the flaws near the girdle facets as shown in Figure 4. In some instances, notably in emeralds, numerous small, uniformly distributed flaws create acceptable gems which appear to “glow” rather than to sparkle. Such numerous inclusions are called jardin, or “garden.”
If cracks or flaws cannot be avoided, the rough should be selected on the basis of (1) uniformity and small size of flaws, and (2) location of flaws near the periphery of the rough gemstone where they can be cut away or hidden beneath girdle facets. Under no circumstances should rough be accepted which contains flaws in the center of the clear area. This rough cannot be used unless it is sliced through the flaw itself and two smaller clean gems made instead of one large one.