Any color undertones - usually black, gray or green - will reduce a stone's value.

Although a pastel stone would be less valued than a deeper blue one, it would be more valuable than a stone considered too dark. In selecting your sapphire, keep in mind that the finest stones are "eye clean", with little or no inclusions (flaws) visible to the naked eye.

Sapphire is readily available in sizes of up to 2 carats, but gems of 5-10 carats are not unusual. The stone is most often cut in a cushion shape - a rounded rectangle - or an oval. But smaller stones are available in round brilliant cuts and a variety of fancy shapes, such as triangle, square, emerald, marquise, pear, baguette, cabochon and others.

Some of the more noted sapphires include the Logan Sapphire, a 423-carat cushion-cut stone from Sri Lanka currently in the Smithsonian Institution in Washington, D.C., and a 258-carat stone set in the Russian crown and kept in the

Diamond Fund in Moscow.

With a hardness of 9 on the Mohs scale, sapphire is harder than any other gemstone except a diamond. This quality makes it extremely durable for everyday jewelry pieces subject to repeated impact, such as rings and bracelets. In general, sapphire can be cleaned with soapy water or commercial solvent and a brush.

It is estimated that about 90% of sapphires on the market today have been heated to maximize their color and clarity. This process is permanent and completely stable.

Perfect natural, untreated gems are exceptionally rare and very expensive. Some colorless or pale stones are treated with chemicals (diffusion treated), which improves the surface color only. This could create a problem if the stone is ever chipped or nicked and needs to be recut or repolished. In addition, some fancy colored sapphire is irradiated to give it a more intense shade. These effects are temporary and can fade in light or heat.