MINERAL OF THE MONTH: October 2008 Mexican Opal

Mexican Opal comes from the Magdalena, Jalisco area of Mexico. Historians have determined that the Aztec people used Mexican opal for ceremonial and ornamental purposes between 1200 and the early 1500s when the Spaniards Conquistadors took over the area. It is known by locals as vitzitziltecpal or the “humming bird stone” due to the iridescent similarity to the feathers of the bird. It also has several other local names including fire opal, jelly opal, crystal opal, cherry opal, and girasol. Many ancients believed that the storm god became jealous of the beauty of the rainbow god. As a result, the storm god broke the rainbow, causing pieces of the rainbow to fall to the Earth to become a part of the opal which exhibits the “rainbow-fire” appearance.

After the Spanish took over the area, the source of these opals was lost until 1840 when Sir Jose Maria Siurab discovered the deposits. Mining still continues today, but the number of opals found has dwindled since the peak production in the late 1960s.

All opals are hydrated silicates containing between 3 and 10 percent water. Some, but not all opals, have a “play of color.” This color show results from diffraction and interference of light rays that travel through the spaces between columns of microcrystaline spheres of silica. The diameter of the spheres within any column determines the resultant color. If there is no color, as is true with common opal, there is a random pattern of microcrystaline spheres, and thus, no structure to act as a diffraction grating.

The specimen featured this month was purchased from a small rock shop located just outside of Yosemite National Park during the late 1990s. I had just finished hiking with my friend, Clare Comstock, when she spotted the rock shop. This six pound gem was on display as an example of opal from a Mexican mine operated by the rock shop’s owner. When I saw the specimen, I asked about its availability for purchase. Although the owner said it was not for sale, she finally agreed to part with it as long as I agreed to display it prominently in the museum. Not only have I done that, but now I’m featuring it as the Mineral of the Month.

I have left this specimen raw because of its innate beauty. Plus, opal can be somewhat fragile and has a tendency to develop fine cracks due to dehydration. I only “dress up” and polish rocks if it will increase the value of the specimen. For this beauty, it is not worth the risk.

Gemological Data

Chemistry: hydrated silicon dioxide
Luster: vitreous to resinous
Hardness: 5.5 – 6
Crystal structure: none, it is amorphous
Fracture: conchoidal to uneven
Cleavage: none
Density: 2.15
RI: 1.42 – 1.43
Birefringence: none

Metaphysical Properties

Opal helps with the amplification of your most positive traits, therefore, providing the ability to overcome your lesser attributes. It can also improve your creative flow and maximize your inspiration and imagination. Opal has been used by the Native American Indians and the Australian aboriginal shaman to invoke visions and to facilitate ceremonial “dreamtime.”

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