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Graining or growth lines are frequently observed in crystals as subtle lines or heterogeneities, caused by slight changes in optical properties of the growing crystal, due to variations in growing conditions, crystal structure defects or other causes. In most cases, graining is only seen with certain orientation of the sample and lighting conditions, in transmitted light and in direction parallel to the crystal face responsible for graining formation.
Internal growth lines in polished diamond. Field of view 2 mm.
Growth lines usually mark certain stages of crystal growth and correspond to different crystal faces formed during the same time. They can be similar to color zoning and phantoms marked by inclusions, forming angles corresponding to the host crystal symmetry. However, some cases can be more complex, as those explained below.
The process of formation of many natural crystals is not a simple continuing process of mineral precipitation. As a result of fluctuations in P-T-X conditions occurring in many deposits, stages of crystal growth can be complicated by periods of dissolution of previously formed crystals, which may be followed by new crystal growth that will fill up dissolved pits and cavities. In such cases, complex growth lines will be observed, marking the border between different growth stages of the same crystal.
Another cause of irregularity in crystals is twinning. Both intergrowth and contact twinning are very common in minerals, and borders between two or more differently orientated crystals can be frequently observed as graining.
Polysynthetic twinning in natural ruby. Crossed polars, field of view 5 mm.
In general, two or more crystals growing together, not only twins but occasionally aggregates, or even one crystal absorbing another of the same species, can produce growth lines in the borders of other gemstones if they are used for cutting or polishing.
Growth lines can be also observed on the surface of a polished gem. They are especially common on polished diamonds. Crystal orientation is extremely important to find the direction for better polishing of the diamond due to its very marked anisotropy of hardness. If the same facet cuts two differently oriented diamond crystals, polishing can be very difficult and may leave a tiny step on the polished surface, since one part of the same facet is harder that the other.
Graininig on the surface of polished diamond. Field of view 1.5 mm.
Growth line going across several facets of polished diamond and lack of polish on one of the facets divided by growth line.
Field of view 1.5 mm.
Also, synthetic gems grown in hydrothermal conditions frequently present a very particular graining. In this case, very fast growth is provoked by a certain crystallographic orientation of the seed plate used for synthesis, which causes formation of large growth blocks slightly disorientated among them, generating a characteristic growth pattern.
Growth structure in synthetic hydrothermal emerald. Field of view 0.5 mm.
Very different, but very characteristic too, are the growth lines observed in synthetic corundums and spinels made by flame fusion (Verneuil) method. In this case, crystals are formed by solidification of melt on a curved growth surface, and this process is reflected in a form of characteristic curved growth lines observed in synthetic Verneuil stones. Sometimes they also can be marked by curved color zoning, as shown in the Color distribution section.
Curved growth lines in synthetic melt grown Verneuil ruby. Methylene iodide immersion, field of view 10 mm.