Friday, 19 April 2013

Overview to Synthetic Rutile (TiO2)

Brilliant cut synthetic rutile; detail enlarged to emphasize its high dispersion.

SYNTHETIC RUTILE (TiO2)


Titanium dioxide. This is produced by a special variant of the Verneuil method, using extra oxygen.

Crystal system


Tetragonal, like the corresponding mineral

Appearance


In the shape of a pedunculated pear or sausage (therefore without obvious crystal faces), generally about 3 centimeters in diameter and no more than 10 centimeters long. It is transparent and usually pale yellow, but can also be bright blue or red; it is not possible to pro-duce a perfectly colorless form.

Physical properties


It has a hardness of 6, which is of course quite low. The density is 4.25 g/cm3. The refractive indices are me 2.61, nr 2.90, thus it is intensely birefringent. The dispersion is also exceptionally high: at 0.28, it is the highest of any stone used for ornamental purposes. It has prismatic cleavage and is brittle.

Production


Appreciable quantities were manufactured in the United States during the 1950s, but production is now very limited.

46.1 Synthetic rutile (or titania)


This was widely used to imitate diamond in the United States in the 1940s and 1950s, but was soon ousted by the appearance of more satisfactory imitations on the 'market.

Appearance


It has very strong luster and is always given a brilliant cut. Normally, it is slightly cloudy rather than perfectly transparent. It has a faint yellowish tinge, but be-cause of its very high dispersion looks positively iridescent in bright light. Under a lens, its very strong birefringence produces an obvious doubling of the facets and edges which is more striking, for instance, than that of zircons. Because the stone is not very hard, the facet edges are not sharp, and it feels slippery.

Distinctive features


Strong birefringence, combined with very high dispersion, low hardness, and a yellow tinge make it easy to recognize.

Cost


Higher than that of other diamond simulants such as cubic zirconia or YAG, but lower than that of the most costly synthetics, such as emerald

Writer – Curzio Cipriani & Alessandro Borelli

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