mygemologists.com

Although many people seem to expect to see a flawless white stone, it may be said that flaws of some kind are almost always present in diamonds, despite the efforts of the lapidary to hide them and the protestations of the salesman to minimize them. The most common faults are small black spots, and these may be present in otherwise good quality stones. If above the girdle, they are more obvious than if below the girdle, but in all instances, value is lessened thereby. Spotted stones are called pique goods in the trade.
Cracks are frequently present, generally very small internal flaws resembling a short length of hair. Light is reflected from such broken surfaces, and these flaws are thus more easily seen in certain directions. Minute cavities or bubbles, which may be empty or filled with carbonic acid, are also not uncommon, but all these faults will not be easily seen except in bad quality stones. Careful examination in various directions is necessary, using forceps or tweezers, not the hands, to hold the stone.

Then we have defects in cutting and polishing which decrease the value of stones. These defects may have a purpose, such as retaining more weight to the detriment of proper shape, or they may be due to lack of skill on the part of the lapidary. Small, unpolished surfaces may be present, or there may be nicks or chips, these often being present in the girdle. Such defects may be minimized if the stone is set in a clever mount. Chips may be due to careless cutting or, in old stones, unskilful unsetting from a piece of mounted jewelry. The careless stocking of loose stones may show in small surface marks, these being caused by the stones rubbing against each other. This holds for all varieties of gem stones. Surface lines left from the polishing wheels are also sometimes seen on the facets.
In addition to these defects we have, of course, the defects of color, and especially a milky hue which clouds some specimens. This is due to irregularity of composition, and it is generally obvious to the naked eye. Each and all of these faults lessen the value of the specimen, but the degree of depreciation is one of current market values and cannot be judged except by those who handle stones daily.
In well cut modern stones, the proportions and facet angles should be approximately those already noted, and all facets in the same set should be alike in size and shape. Therefore, each large facet of the stone should be patiently examined, both from the back and the front. But it should be realized that very few stones are perfectly flawless. In practice, small differences in shape are often not seen, weight being retained to the detriment of good proportion.

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