Gem-quality

All static studies at pressures beyond 25 GPa are done with diamond-anvil cells conceived independently by Jamieson [32] and by Weir etal [33]. In these variants of Bridgman s design, the anvils are single-crystal gem-quality diamonds, the hardest known material, truncated with small flat faces (culets) usually less than 0.5 nun in diameter. Diamond anvils with 50 pm diameter or smaller culets can generate pressures to about 500 GPa, the highest static laboratory pressures equivalent to the pressure at the centre of the Earth. 

Diamond is crystallized in cubic form (O ) with tetrahedral coordination of C-C bonds around each carbon atom. The mononuclear nature of the diamond crystal lattice combined with its high symmetry determines the simplicity of the vibrational spectrum. Diamond does not have IR active vibrations, while its Raman spectrum is characterized by one fundamental vibration at 1,332 cm . It was found that in kimberlite diamonds of gem quality this Raman band is very strong and narrow, hi defect varieties the spectral position does not change, but the band is slightly broader (Reshetnyak and Ezerskii 1990). 

Some piezoelectric crystals are electrically polarised in the absence of mechanical stress one example is gem-quality tourmaline crystals. Normally, this effect is unnoticed because the crystal does not act as the source of an electric field. Although there should be a surface charge, this is rapidly neutralised by charged particles from the environment and from the crystal itself. However, the polarisation decreases with increasing temperature and this can be used to reveal the polar nature of the crystal. If tourmaline is heated its polarisation decreases and it loses some of its surface charges. On rapid cooling it has a net polarisation and will attract small electrically charged particles such as ash. Such crystals are known as pyroelectric, and ferroelectric crystals are a special subclass of pyroelectric crystals. 

The crystal figures shown in Fig. 9.2 were selected from sketches that appeared in a series of books published between 1913 and 1923, and all represent forms of single or twinned crystals larger than a few millimeters. They are mostly gem-quality diamonds. Other than these forms, there are translucent or opaque... 

The first and the most typical type is called coated stone, which has a clear, gem-quality internal portion, coated by a mantle portion with fibrous texture. In appearance, this type of diamond is opaque and of industrial quality, but if a window is made the clear, gem-quality interior is discernible. Coated stones have the following growth history. 

Optical. Besides the gem qualities dependent on optical properties, diamond is very useful as a light-transmitting window for lasers and for simple windows for monitoring chemical processes in corrosive and otherwise hostile environments. 

Research. A significant impact on research at high pressure has come about with the use of gem quality diamonds as Bridgman-type anvils in a small compact high pressure device (40—42). With this type of apparatus, pressures greater than those at the center of the earth (360 GPa = 3.6 Mbars) have been reached, and phase transformations of many materials have been studied. Because of the x-ray transparency of diamond, it is possible to determine the structure of the phases under pressure. Because of the strenuous environment, crystals selected for this application have to be of very high quality. 

Gem quality beryls are aquamarine (blue), emerald (green), and golden beryl. Likewise, amethyst b a violet-colored silica, and sapphire (blue) and ruby (red) are alumina. Yet pure beryl (Be,AI>Si lO ). alica (SiO ). and alumina (AI O>) are colorless. Explain... 

Ill the United Slates, common corundum is found in New York. New Jersey, Pennsylvania. Virginia. North Carolina. South Carolina and Georgia sapphires of gem quality near Helena. Montana, associated with alluvial gold in the Missouri River. From the crystalline limestones and schists of the islands of Naxos and Samos in the Grecian archipelago most of the emery of commerce comes. Other deposits are near Ephesus in the Middle East, and in the mwn of Chester in Massachusetts. The word corundum conies from the Hindu, kurand emery is derived from the Greek name for this substance. 

Gem quality crystals from Ceylon (Sri Lanka) have been known for many years. They range from colorless to brownish orange, yellow, dark red, from light reddish-violet. Heat treated zircons provide a beautiful stone of light blue color. Colorless stones are used as a diamond substitute. 

Gem is a general term used to describe precious and semi-precious stones, usually after they have been cut and polished. In archeology, it refers only to engraved stones (cameo, intaglio, seals, etc.). The term gem-quality is applied to rough material that is of a quality suitable for cutting or working... 

Jade is the common name for gem-quality specimens of two distinctly different mineral species, jadeite and the massive variety of actinolite, called nephrite. The word jade is derived from the Spanish piedra deyjada, meaning stone of the flank. This refers to its popular use as a cure for diseases of the kidneys and liver. Other minerals that have been mistaken for jade, or used as jade imitations include green jasper (quartz), vesuvian (idocrase), massive grossular garnet, chloromelanite (a mixture of dark pyroxenes) (Table 2.10). 

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