Sapphire Crystals:

Oscar Waldan on the issue of sapphire crystals:

Sapphire crystals are composed of Corundum. In the early 1960's, they were being used for ladies watches and were faceted. These were used in very small faced ladies watches and were very expensive to produce.

In the early to mid 1980's, sapphire crystals were beginning to be offered more for men's watches. I was one of the first to use sapphire crystals in about 1983 or so. Subsequently, Rolex began using sapphire crystals in the mid to late 1980's. On a further note, I use thick contoured sapphire crystals which cost about 3.5 times more than flat sapphire crystals. Many companies producing watches that cost thousands more than mine use flat sapphire crystals as a cost cutting method.

Sapphire crystals are currently the best and most durable material available for watch crystals. They are synthetically made and rate 9 on the Moh's scale of hardness. The only thing harder, and capable of scratching a sapphire crystal, is a diamond which rates a 10 on the Moh's scale of hardness. Synthetic Sapphire (AL₂O₃), is produced from aluminum oxide. It is much less expensive than natural sapphire but is absolutely identical regarding structure, properties, and quality.

Our contoured sapphire crystals are high polished and burnished in to the case. That is, they are specially attached to the case by a machine under pressure and there is a gasket between the case and the crystal thereby making the case water resistant. It is similar to a hermetic seal. Case and crystal measurement tolerances are very high in order to achieve this type of seal.
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Definitions of Sapphire and Corundum:

Sapphire: transparent to translucent, natural or synthetic variety of corundum (aluminum oxide, Al₂O₃) that has been highly prized as a gemstone since about 800 BC. Its colour is due mainly to the presence of small amounts of iron and titanium and normally ranges from a very pale blue to deep indigo, with the most valued a medium-deep cornflower blue. Colourless, gray, yellow, pale pink, orange, green, violet, and brown varieties of gem corundum also are known as sapphire; red varieties are called ruby. Much sapphire is unevenly coloured; it is also dichroic; that is, the colour of most varieties changes with the direction of view. Alexandrite sapphire appears blue in daylight and reddish or violet in artificial illumination, somewhat like true alexandrite. Careful heating and cooling under various conditions can induce permanent colour changes in sapphire (e.g., from yellow to colourless or greenish blue and from violet to pink). Other colour changes result from exposure to intense radiation. Most sapphire contains abundant microscopic inclusions; reflections from these yield a faint whitish sheen, known as silk. Tiny, regularly arranged mineral inclusions (commonly rutile) and elongate cavities are responsible for the asterism shown by star sapphire.

Sapphire is a primary constituent of many igneous rocks, especially syenites, pegmatites, and various basic (silica-poor) types; it also occurs in schists and metamorphosed carbonate rocks. Most commercial production has come from alluvial gravels and other placer deposits, where the sapphire commonly is associated with ruby and other gem minerals. The best known sources, including some lode deposits, are in Sri Lanka, Myanmar (Burma), Thailand, Australia (Victoria, Queensland, New South Wales), India, Madagascar, Russia, South Africa, and the United States (Montana, North Carolina).

Most transparent sapphire is faceted, generally in the brilliant style. Such gems have considerable sparkle, but they exhibit little fire because of their modest dispersion (separation of light into its component colours). Skillful cutting of unevenly coloured stones yields gems with a uniform appearance derived from only small portions of relatively deep colour. Star sapphire and other nontransparent varieties are cut en cabochon (in convex form, highly polished) rather than faceted. Despite its great hardness, some sapphire is carved or engraved, especially in the Orient.

Synthetic sapphire has been produced commercially since 1902. Clear, sound material is manufactured in the form of carrot-shaped boules and slender rods. Much is consumed by the jewelry trade, but most synthetic material is used for the manufacture of jewel bearings, gauges, dies, phonograph-needle points, thread guides, and other specialized components; some also is used as a high-grade abrasive. Synthetic star sapphire is made with luminous stars that are more regular and distinct than those in most natural stones; the asterism is obtained through controlled exsolution of impurities.
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Corundum:
naturally occurring aluminum oxide mineral (Al₂O₃) that is, after diamond, the hardest known natural substance. Its finer varieties are the gemstones sapphire and ruby, and its mixtures with iron oxides and other minerals are called emery.

Corundum in its pure state is colourless, but the presence of small amounts of impurities can impart a broad range of hues to the mineral. Ruby owes its red colour to chromium, sapphire its blue shades to the presence of iron and titanium; most corundum contains nearly 1 percent iron oxide. The mineral readily weathers to other aluminous minerals, margarite, zoisite, sillimanite, and kyanite.

Corundum crystallizes in the hexagonal system, forming pyramidal or rounded barrel shapes. It is widespread in nature, being found in igneous, metamorphic, and sedimentary rocks. Large deposits are rare, however. Some of the richest deposits occur in India, Russia, Zimbabwe, and South Africa.

In addition to its use as a precious gem, corundum finds some use as an abrasive, owing to the extreme hardness of the material (9 on the Mohs hardness scale). It is used for grinding optical glass and for polishing metals and has also been made into sandpapers and grinding wheels. Because of its high melting point (2,040°FC, or 3,700°FF), it has also been used in refractories.

In most industrial applications corundum has been replaced by synthetic materials such as alumina, an aluminum oxide made from bauxite. Artificial corundum may be produced as a specialty product, as for gem use, by slow accretion and controlled growth on a boule in an oxyhydrogen flame. This procedure is known as the Verneuil process.
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Source of definitions: http://www.britannica.com