During the growth of any kind of gem material in nature or in the laboratory, foreign materials or smaller crystals of the same material are often caught up in the accumulating matter, or structural irregularities appear. If the gem materials is transparent and if the enclosure is large enough, it may be visible to the unaided eye or under sufficient magnification. materials caught up during the growing process are called INCLUSIONS. Fissures, cleavages, fractures and nicks are more accurately described as IMPERFECTIONS, FLAWS or BLEMISHES.
Rather then regarding inclusions in colored stones as harmful, in small sizes and numbers that do not detract in, any way from their beauty they should he regarded as adding to desirability, for they provide identifying characteristics. Not only do they separate that stone from any other stone of the same variety, but in the natural they prevent confusion at a later date with what synthetic materials may be available. Dealers tend to regard colored stones as perfect when they contain no flaws that are visible to the unaided eye. In the more expensive qualities of the more valuable gemstones, flawlessness under 10x is all but unknown. In addition, since ruby, sapphire and emerald are synthesized in large quantities, it is better for the fine natural stories to contain identifying, characteristics.
Inclusions may be solid, liquid or gas or combinations thereof. Solid inclusions may be of the same material but oriented or coated in such a manner that they remain visible, or they may be of any foreign matter that was present during their growth. Liquid inclusions usually, but not always, contain gas bubbles. The liquid is generally of the same composition as that of the solution in which the crystal grew. Since natural gemstones and almost all stones synthesized by the verneuil process (corundum, rutile, spinel and strontium titanate) may contain gas bubbles that were caught up in the molten materiel as it solidified. Glass, both natural and artificial, as well as amber, may also contain spherical or elongated gas bubbles. Natural crystalline gemstones do not contain spherical gas bubbles, except in angular cavities containing both liquid and gas; however, some stones, such as diamond, have white, cloudy inclusions that may be gas filled enclosures.
Another characteristic usually considered under inclusions is color banding. In addition, twinning planes, which show up as discontinuities in a stone, are considered under inclusions. The same is true of growth lines such as the striate seen in synthetic corundum and the swirl marks in glass.
A knowledge of the difference between the appearance of inclusions resulting from artificial manufacture and the inclusions of other minerals or liquids that occur in genuine stones is necessary to become an expert in the detection of synthetic gem materials.
Inclusions and growth structures will be given more detailed consideration under the various gemstones later in this course.