Synthetic quartz is grown by the hydrothermal process and is used primarily for scientific and industrial applications in the United States. Recently, commercially available colors of blue, green, purple and yellow from Russia have been noted. Japan is also reported to be producing colored synthetic quartz. The coloration agent for the dull blue is due to cobalt; the green is due to iron, resulting in a rather grayish tourmaline green. Early endeavors in its manufacture revealed that the crystallographic orientation of the seed plate was paramount in obtaining a sufficient quantity of growth. In general, the faces of a crystal represent directions of slowest growth, while edges or comers grow faster, with the greatest growth rate taking place parallel to the C-axis.
For growth, high temperatures plus high pressure water solutions are employed. The size of the finished crystal depends solely upon the size of the seed crystal used, and crystals weighing up to two pounds have been noted. This seed plate is cut from clear un-twinned quartz approximately 2 mm in thickness. When viewing the finished crystal parallel to its side, a clear band is seen cutting through the center, surrounded by a top and bottom of synthetic quartz growth that extends all the way to the surface. The surface of the blue crystals are very glossy, while the yellow and green materials are usually frosted in appearance. All types are characterized by innumerable small conical mounds, each of which is the locus of the growth spirals by which successive layers of atoms are added to the growing crystal surface (see Figure 11).
When dealing with the separation of synthetic from natural quartz, there is no appreciable difference in the density and refractive indices. The synthetic material is, in general, un-twinned, unstrained, and free from all but very minute inclusions. These inclusions are two-phase-gas and liquid. This reflects the fact that liquid has contracted more than the solid quartz material in the growth process, leaving a bubble of water vapor with liquid. Solid inclusions are rare, but occasional strain cracks can be found running at angles from the colorless band of the seed crystal (see Figure 12).
A spectroscopic examination of the blue material reveals one main broad band centered near 6500 A.U., with others at 5900 and 5500 A.U.; there is also a very diffused band between 4900 and 5000 A.U., which is usually only visible in a long section of the material (see Figure 13). The remaining colors show no characteristic absorption spectra.
