Corundum

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Chemical composition -- Aluminum oxide.

Color -- almost all colors. Medium to dark red stones are known as ruby; other colors, sapphire.

Optics -- R.I. 1.76-1.77.

Durability -- Hardness 9. After diamond, the next hardest gem material. Tough, but still subject to chipping and abrasion. Stones worn regularly in rings will periodically need repolishing to remove minor chips and pits.

Crystal structure -- Hexagonal .

Specific Gravity -- 4.0.

Sources -- Many locations, but the best known are in southeast Asia, Australia, and southern Africa. In the U.S., Montana and North Carolina are well known sources.


Ruby

Perhaps the most prized of gemstones in either transparent faceted form or chatoyant star rubies. One of the prime sources is the Mogok area of Burma. Fine stones over 2 carats are rare and expensive.


Sapphire


One of the most popular gemstones in its blue colors. Very dark grayish or greenish stones are abundant, as are heat-treated stones. Grayish "geuda" is routinely heat-treated to improve color and clarity and provides the bulk of the sapphires seen today. Fine blue stones over 5 carats are rare and valuable. Another highly prized variety is the medium pinkish-orange padparadscha. Ceylon (Sri Lanka), Burma, Thailand, Australia, Montana, and the Umba Valley region of east Africa provide a wide variety of colors.

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