Gemstones Generally
There are a limited number of elements and chemical compounds that have the physical characteristics to be useful as gemstones. The physical characteristics that are generally accepted as being most important are hardness, refractive index and color, although thermal stability, chemical stability and toughness are also considered important in many gemstone applications. To date, the only chemical substances technically considered precious stones are diamond (single crystalline carbon) and corundum (sapphire and ruby single crystalline aluminum oxide!) because their hardness when measured on the Mohs scale is approximately 9 or higher. The Mohs system is a scale for ranking the hardness of a mineral with diamond being the hardest at 10, sapphire at 9, topaz 8 down to the softest mineral, talc, which is 1. Emerald, because it is rare, is accepted as a precious stone even though its hardness is 7.5 while other gems, such as chrysoberyl, topaz and garnet, are usually classified as semiprecious stones because of their lower hardness. Hardness has practical value in that it defines the ability of a gemstone to resist scratching.
Refractive index is important because it defines the ability of a gemstone to refract light. When materials with a high refractive index are fashioned into finished gemstones they appear brilliant when exposed to light. The characteristic brilliance of a diamond is due mainly to its high refractive index.
The color of a gemstone is determined by a variety of factors, from the impurity atoms that are available to be incorporated into the crystal lattice to the physical and electronic structure of the crystal itself. A ruby, for instance, is simply a sapphire crystal (aluminum oxide) that contains a small concentration of chromium impurity atoms.
The thermal and chemical stability of a gemstone can be important during the process of mounting stones into jewelry. In general, it is beneficial if stones can be heated to high temperatures without changing color or reacting with ambient gases (that mar the surface finish).
The toughness of a gemstone relates to the ability of the gemstone to absorb energy without breaking, chipping or cracking. A gemstone must be able to withstand those impact forces normally encountered during a lifetime of use mounted on a ring or other jewelry item.
Hardness, refractive index, color, thermal/chemical stability and toughness are all characteristics that, in combination, determine the usefulness of a material as a gemstone.
Synthetic Diamond Gemstones
Dating from the 1950s, an effort to produce gem-quality synthetic diamonds was pursued by General Electric Company as evidenced by numerous patents, including U.S. Pat. No. 4,042,673. These efforts centered around the use of very high pressure/high temperature environments for growth of monocrystalline diamonds on seed crystals. Gem-quality synthetic diamonds generally have not gained commercial acceptance.
Synthetic Diamond Films and Coatings
Synthetic diamond films and coatings are now available for certain applications. These films may be grown from the vapor phase by either chemical vapor deposition (CVD) or physical vapor deposition (PVD), with CVD being more prevalent in its use. The deposition of a synthetic diamond film on a substrate using CVD requires an activated gas phase that is activated by high temperature and/or plasma excitation, with the gas phase including a carbon-containing species such as CH.sub.4, CO, CO.sub.2, a hydrocarbon or an alcohol. Since the gas phase so described will tend to deposit both diamond and graphite on the substrate, the gas phase must also include a species such as atomic hydrogen that preferentially etches graphite. The CVD process also requires a substrate surface receptive to nucleation of diamond thereon and a temperature gradient between the gas phase and the substrate to drive the diamond producing species to the substrate.
Commercial uses of diamond films and coatings are largely limited at present to non-electronic application such as abrasives, machine tool coatings, x-ray detection windows, IR optical coatings or elements, and electronic heat sinks.
Synthetic Silicon Carbide Used As Abrasives And Semiconductor Materials
Silicon carbide is rarely found in nature. However, it has been manufactured for more than eighty years, in crystalline form, for abrasive products. Silicon carbide crystals found in nature and in abrasive products are black and not transparent because they contain substantial levels of impurity atoms.
During the 1960s and 1970s, significant development activities were initiated with the objective of growing large (bulk) crystals of low impurity silicon carbide for use in the production of semiconductor devices. These efforts finally resulted in the commercial availability of relatively low impurity, transparent silicon carbide crystals in 1990. These silicon carbide crystals are only fabricated and marketed as very thin, green or blue (175 um-400 um) slices useful for semiconductor devices.
Silicon carbide has a very high hardness (9.25-9.5 Mohs depending on the polytype atomic arrangement! and crystallographic direction)and a high refractive index (2.5-2.71 depending on the polytype). Furthermore, silicon carbide is a very tough and an extremely stable material that can be heated to more than 2000.degree. F., in air, without suffering damage.
Silicon carbide is a complex material system that forms more than 150 different polytypes, each having different physical and electronic properties. The different polytypes can be classified in three basic forms, cubic, rhombohedral and hexagonal. Both the rhombohedral and hexagonal forms can occur in a number of different atomic arrangements that vary according to atomic stacking sequence.