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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.
Here are some of the technical characteristics of Sapphire Crystal:*
Physical Properties:
Crystalline Structure: Rhomboedral hexagonal single crystal
Density: 3.98 gm/cm3
Composition: AL2O3
Thermal Properties:
Melting Point: 2320°K 3716°F 2047°C
Softening Point: 2070°K 3266°F 1797°C
Mechanical Properties:
Hardness: 9 Mohs 2500-3000 Vickers 1800-2200 Knoops
Optical Properties:
Transmission: Visible Light >> Excellent
*Specs. obatined from Stettler Saphir Tech Info Library
Link đọc thấy rất thú vị và chi tiết về kính sapphire này:
http://www.pmwf.com/cgi-bin/ForumArchive/webbbs_config.cgi?noframes;read=273655