Opal This essay discusses some characteristics and history of the opal.

Essay by mr.moeHigh School, 11th gradeA+, December 2004

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Opal is the modern October birthstone and the accepted gem for the 13th wedding anniversary. Most opals are between 50-65 million years old, dating back to the Cretaceous period when dinosaurs roamed the earth. Opal formed as silica from decomposing rocks mixed with ground water which formed a silica gel that collected and hardened in underground cavities and fissures.


Black Opal comes from the Lightning Ridge region of Northern New South Wales. It is the most rare and most valuable opal. Fine quality black opals often cost more per carat than diamonds.


Light opal is the most common type of opal found and refers to both the crystal type opal (translucent) and the milky opal (opaque). It is the most common variety and is generally less valuable than gem quality light opal. Brilliantly colored light opals may be quite expensive with a value exceeding some Black and Boulder Opals.

Brilliance or luster is one of the primary factors that determines an opal's value. An opal with strong intensity and color adds more value.

The hardness af an opal is between 5.5 and 6.5 on Moh's scale.

Most opals are found where geothermal hot springs dried up during seasonal periods of rainfall...