In his efforts to identify gem materials, the gemologist is assisted by the refractometer, polariscope, microscope and spectroscope. Usually, he is able to classify stones according to species on the basis of polariscope and refractometer reactions. However, synthetics and other substitutes that may be closely related in properties and other characteristics to the gem imitated may require other means for identification. In this instance, inclusions seen under magnification may be the deciding factor.
Foreign substances that have been trapped during the growth of a gem mineral are not the only features that may be included under a broad definition of the term inclusions. Flaws and blemishes in the form of nicks, scratches, fractures and cleavages may also be considered; they, too, prove useful in identification when viewed under magnification by a competent gemologist. Moreover inclusions often give the trained man an idea of the conditions under which a gemstone formed. Sometimes the nature of these impurities are characteristic of a certain deposit; therefore, they may suggest or even prove the source of a given stone. Principally, however, the value of inclusions is to ascertain the general method of formation; that is, whether a stone was formed in nature or by man.
Formation of Inclusions
Solid inclusions usually formed before the growing host mineral enclosed them. For example, pyrite crystals were trapped in emerald as the emerald crystal grew. However, some inclusions form after the gem was crystallized. A good example of solid inclusions crystallizing within a solid gem material is the growth of rutile needles in synthetic star corundum. To produce the star causing needles, boules with a small titanium-oxide content are subjected to a long period of reheating, during which the titania crystallizes in a pattern determined by the crystal structure of the synthetic corundum.
It is only in rare circumstances that crystal growth occurs under such ideal conditions that transparent materials result. It is even rarer when completely flawless materials develop. Atoms of the essential constituents start to accumulate in the expected manner at many points when conditions for crystal growth are reached. These blocks of crystal material (actually tiny crystals) joining together in mossaics to form larger crystals. Often impurities between adjoining blocks prevent perfect alignment. Many such discontinuities add up to grossly imperfect structure and non-transparency.
Gem materials may crystallize from silica melts, aqueous solutions or gaseous solutions, or they may form from re-crystallization in the solid state as a result of reactions between the solid and fluids or between solid and solid. Crystals usually start to grow when conditions in their environment change or reach a stage at which crystallization is encouraged. Usually changes in temperature or pressure trigger crystallization, but changes in chemical environment also may cause it. Growth usually is controlled by the complex interaction of many factors.
The growth of transparent crystals of many minerals in the laboratory taught us a great deal about the conditions that existed at the time of formation in nature. As crystallization begins, it is apparent that many crystal nuclei are formed that consist of fragments in which atoms of the gem material have taken their proper lattice positions for the gemstone. Such nuclei usually are just a few atoms in width, but they may be considerably larger. During the initial stages of growth, the conditions that exist usually are reversible; i.e. although a net increase in the number of atoms takes place, some atoms join and others detach as the crystal begins to grow. Since atoms are both joining and departing from the surface, some tiny nuclei are destroyed by the removal of just a few atoms, while others are growing. If a few atoms detach before the nuclei are large enough, they are destroyed. Under favorable conditions once the critical stage is passed when the departure of a few atoms means destruction, growth continues.
Although it is possible for crystals to grow by the addition of individual ions or atoms, the usual situation is that atom groups of various sizes that have arranged themselves in the expected pattern join on to the growing crystal. In other words, the accumulation process is likely to take place throughout the growth area, that the unit joining the growing crystal may vary from several atoms or ions to several million. During such growth, it is usual for the joining material to orient itself with aspect to the crystal nucleus, if conditions of growth are reasonably good. If growth is rapid and the material that is being joined to the growing crystal is oriented differently and does not reorient itself, the result will be an included crystal of the same material. If it is large enough, such an inclusion is visible and apparent as an included crystal, because its orientation is unlike that of the host. Obviously, crystals of other materials that are forming in the solution would be apparent within a transparent host material if large enough. They would be obvious because of the differences, in and other properties.
The transparency and perfection of a crystal depends on the order lines of the atoms and groups of atoms that have been added to the original unit. Usually, if crystals grow very rapidly, they enclose many materials that do not fit the structure of the growing crystals. Such crystals are likely to be opaque, either because too many materials have been enclosed that do not fit the structure or because of gross irregularities in structure.
Liquid and gas inclusions are likely to be found in voids in the crystal. Usually, under the conditions of higher temperature at which the crystal formed, liquid filled the whole space (Figure A) but later, a gas bubble separated out as the liquid cooled and contracted (Figure B). Crystals also may form in the liquid; for example, the well known three phase inclusions of Colombian emeralds, consisting of tiny crystals, gas and liquid in the cavities. Three-phase inclusions have not been reported in other materials, but two-phase (liquid and gas) inclusions are very common. In natural crystalline materials, gas bubbles surrounded by a solid, such as those trapped during the formation of boules by the Verneuil process, are never seen. During this process, which is unlike anything in nature, the powdered aluminium oxide is dropped through a flame and accumulates on a rotating refractory pedestal. The material crystallizes because corundum and spinal have a high tendency to do so; however, crystallization sometimes takes place before the bubble caused by boiling escapes.
Typical Inclusions in Artificial Materials
As a general statement, it may be said that glass and those synthetics formed by the Verneuil process are characterized by inclusions or markings of a curved nature. Under magnification, glass may have a swirl effect similar to that produced by stirring a concentrated sugar and water solution. Curved accumulation lines are characteristic of synthetic ruby, synthetic alexandrite like sapphire and some synthetic blue sapphires. However, the most highly diagnostic inclusion in glass and flame-fusion synthetics is the spherical or nearly spherical gas bubble. Bubbles in synthetic often have "tails" caused by movement during the instant before a layer hardened. In glass, often they are stretched into a marquise shaped outline sometimes, oval bubble are encountered also. Bubbles formed by boiling at the top portion of the boule tend to rise to the surface; thus, the tails usually are at right angles to the direction of the curved striae that conformed to the slightly curved surface of the boule top. Bubble in glass sometimes are stretched intentionally during the final stages of formation, so as to give the impression of silk like inclusions. Glass cat's-eyes with a sharp eye have been made by this method. At times, the bubbles in synthetic spinel assume a crystal shape; usually, they are nearly round, but they may have many slightly flattened faces.
Angular inclusions may be encountered occasionally in both glass and flame fusion synthetic corundum and spinal; they are caused by various factors. Under certain circumstances, crystallization may start during the cooling of glass and result in angular crystals. An example of this is a certain type of light-blue glass resembling aquamarine that contains large arboritic crystalline forms. Also, efforts are sometimes made to simulate natural inclusions by introducing materials that will not melt at the temperature at which glass forms. When such inclusions are encountered, bubbles are almost always present in great numbers. Angular inclusions in Verneuil synthetics are seen only in those stones that were made in the early days before the fed mechanism on the furnace was perfected; as a result, the aluminum oxide powder fell too rapidly to be melted fully and crystallized before the next fall of powder. With these exceptions, glass and Verneuil synthetics are characterized by rounded inclusions.
Synthetic materials grown by flux fusion or the hydrothermal process are not so easily classified, for they lack the rounded inclusions of the flame fusion products. Since they are formed in a manner similar to nature's methods, angular inclusions are often present. Ardon and Chatham synthetic ruby, as well as Linde, Gilson, and Chatham synthetic emerald are characterized by inclusions that appear very similar to those In natural stones; however, there are distinguishing features in these synthetics that permit thorn to be identified readily.
The Lechleitner synthetic emerald shows internal characteristics that are unique, because of its unusual structure. Since the beryl seed to which the synthetic emerald overgrowth is added comprises the bulk of the stone, natural inclusions typical of beryl are to be expected. The overgrowth is very thin and appears to have been subjected to checking; as a result, there are many parallel cracks in the coating (Figure C).
Often, a cross-checking approximately at right angles is also evident (Figure 4). The latter is not as likely to be arranged in parallel lines as is the first set of cracks. Although these are reasonably diagnostic features, the easy method of identification is by immersion.
Inclusions as Diagnostic Features
Inclusions may be sufficiently diagnostic to provide almost certain proof of the nature of some gem materials. Usually, however, the competent gemologist uses inclusions only for indications of identify, rather than as the sole basis for a decision. For example, although it may be said almost without equivocation that a green stone of high luster with brown fiber like inclusions is demantoid garnet, the possibility of encountering similar inclusions in another green stone always exists; therefore, the gemologist should be sure that the stone also has a very high dispersion and is singly refractive before completing the identification.