The Opal -- Beautiful Survivor
by Margaret Burgon Klemp
There is the precious opal, the common opal, the fire opal, the Peruvian opal and then, of course, there is the synthetic opal. The Sanskrit language knew the stone as upala, while the Greek variation is opallios while the Latin world for opal is opalus. All of the different word origins refer to the special nature of the opal. The Sanskrit word literally means "precious stone," and the Greek and the Roman meanings have a shared outcome. They called the opal "precious stone".
There are a lot of different trade names for opals, but the most recognized and widely used is the precious and common opal. Even though the opal is brittle and can break quite easily, and is prey to scratches and heat it remains high on the list of gemstone collectors. The reason is that opals are truly beautiful. Its. beauty fascinated earlier cultures. Some even believed it fell from heaven. The birthstone of October is the opal and in the 19th century those who were superstitious believed that only those born in that month should wear opals. Anyone else wearing one was supposed to have run the risk of having bad luck or running into a dire fate.
An opal is actually what is known as a mineraloid, a shapeless gel with silicium dioxide that was deposited at a low temperature in crevices and fissures of all different types of rock. The most common types of rock where they were deposited are limonite, sandstone, rhyolite and basalt. It can also form original fossils or replace fossils that have already been formed, and collectors find this phenomenon very interesting. Opals are relatively young gems, and they are so fragile they could never have survived early geological upheavals that produced other types of stones. Opals did occur in volcanic masses of calcite that filled the veins in lava rock. Hot, briny gel like mixtures would rise up to the surface from the volcano which decomposed the calcite. What was left behind was colloidal silica, and this along with water later produced opals.
The precious opal has a unique internal structure. It is made up of spheres that are closely packed in layers that eventually form something that looks like a four-sided triangle or a hexagon. They are made up of silica that allows light to pass through the structure like a prism to produce internal colors, and it also determines the quality of the stone. A precious opal has a doublet which is a colorful material laid on a black backing of ironstone, basalt or obsidian. This causes "a play of color" which produces a spectrum of different colors.
The common opal is quite a departure from the precious opal. There are several different types of common opals: the milk opal, opals with blue and green tints, resin opal, and a wood opal. There are also opals that can be found around hot springs and geysers. They are a clear glass opal made up of geyserite. There are other varieties of opals most notably the fire opal and Peruvian opal. Mexico and Mesoamerica is the chief source of the fire opal. They come from the Mexican high country where they can be found in rock surrounding extinct volcanoes. It is extracted from open-pit mines in the canyons and valleys there. It should be noted that more durable fire opals are found in drier region. The drier the region the more durable the opal will be. While Mexico produces the most fire opals they can be found in Honduras, Ethiopia, United States, Australia, Canada, Turkey and areas in former Russian states.
The Peruvian opal is said to aid sleeping and calming down the mind. It is a blue opal that is quite rare and can only be found in the Andes Mountains in Peru. It has a Caribbean color to it, and is considered the national gemstone of Peru. Charlatans have taken to pushing dyed blue opals onto the market. Experts say that if the blue color of the stone appears to be uneven then it is probably a dyed stone and not a valid opal.
Microcrystalline opals are made up of a microcrystalline material which is a crystallized substance or rock which contains small crystals that are visible only through microscopic examination. These opals are broken into two categories: Opal-CT is where a predominant mineral rich layer is present within the opal layers. They are all genuine minerals that formed from evaporating water with low amounts of soluble silica. They are not Opal-A because the A type of opals thrived during dry spells with very little water input.
Opal-C is made up of an arrangement known as a-cristobalite which is formed into a number of disordered stacks. Cristobalite is found in volcanic rock where the temperatures were high and the succeeding environments were arid.
Non-crystalline opals have two categories designated as Opal-AG and Opal AN, but they are usually grouped together as Opal-A. Non-crystalline is amorphous without real or apparent crystalline form.
The difference is that AG is a group of spheres of silica that has water filling the gaps. Both precious and common opals are signified as AG. The AN label refers to an opal that was formed in water containing amorphous silica-glass.
The Opal is the national gemstone of Australia. The state of Nevada in the United States honors the black opal as its' state gem stone. The black opal is found in Humboldt County, Nevada.
References:
Gems: A Lively Guide for the Casual Collector, by Daniel J. Dennis Jr., Henry N. Abrams, Incorporated, New York, New York
Gemstones: Symbols of Beauty and Power, by Eduard Gubelin and Franz-Xaver Erni, Geoscience Press, Tucson, Arizona
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