Thursday, October 13, 2011

Rock Review: Opal

Today’s Rock Review highlights the Opal, October’s birthstone.

Geology Buzz
Opal gets its name from the Latin word, “Opalus” meaning precious jewel. Opal is a non crystallized version of Silica that formed where ancient boiling hot springs existed in prehistoric times. As the springs dried up, Silica chemically combined with water and became lodged in cracks of the earth’s surface. Opal’s actually still contain some water, up to twenty-one percent. That is why it’s important to carefully guard it from heat and chemicals that can react with the water, causing cracking and loss of iridescence. It is a five to six on Mohs scale of hardness, so it must be shielded from blows as well.
The structure of the Opal is what makes it diffract light and have its unique internal play of iridescence. It consists of Silica spheres that exist in hexagonal or cubic lattices. The conditions under which an Opal forms determines its color. Most Opals are a mottled clear to gray, however, more exotic colors such as red, blue, pink, green, yellow, brown and black have been found as well, black being the rarest.
Fun Facts
Legend has it that the Opal fell from heaven with a bolt of lightning. Ancient cultures valued Opals for their ability to ward off evil and guard eyesight from damage. Nobility often wore them in crowns and as necklaces for security. They were even used medicinally ground up and ingested for healing and to protect from nightmares. Thankfully we don’t use our Opals in this way anymore.
Ninety-seven percent of the world Opals come from Australia, which is why it’s the official gemstone of the country. More recently, common Opals have begun to be valued for use in the jewelry market as well as precious Opal. Some fun common Opal options to consider are the Milk Opal in milky white and the Peruvian Opal in pink and blue. There is less iridescence in common Opals, but still some unique mottled colors that make them quite beautiful. Giving Opals as a gift symbolizes faithfulness and confidence. What a great message for lucky Birthday guys and gals this month!

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