![opal birthstone opal birthstone](http://www.swedesjewelers.com/images/store-pics/october-birthstones-opal-tourmaline-1280x550.jpg)
It has a conchoidal to splintery fracture and is often very brittle. Opal is neither very hard (5 1/2 to 6 1/2 on a scale of 10) nor very tough. Play of color is sometimes erroneously called "fire." The term "fire opal" should be used to describe opals with a red body color they may or may not show play of colors. When the packing of the silica spheres becomes faulted, the striations produce colors that can be seen in reflected light. The spheres are arranged in octahedrons (base to base pyramids). The play of color is caused by diffraction of light by the ordered arrangement of closely packed microscopic silica spheres with water enriched spaces between them. Precious opal, or opal with a play of color. Fibrous microcrystalline quartz, microcrystalline opals and non-crystalline opals all showed diffuse X-ray small angle scattering that were due to inhomogeneous distributions of water in their microstructures. They listed seven species of opal and characterized the microstructure of each. Researchers in Germany have produced a detailed nomenclature of micro- and non-crystalline silica minerals. The water is neither a water of hydration as the first formula suggests nor is it unattached to the silica as the second formula might imply. The formula of opal is often written as either SiO 2.nH 2O or SiO 2 + nH 2O. Silicon dioxide, SiO 2, the same elements that make up quartz is also the constituent of opal. That is because opals are unstable and are just one of the phases through which gel-like silica (SiO 2) can pass on its way to becoming stable crystalline quartz. The last observation may have actually been based on a fact: opals can deteriorate and change from a highly colorful, somewhat glassy stone to a rather colorless mass of a chalky silicon dioxide. When Anne died, the opal faded and lost all of its color. It seems that Anne had an opal that that reflected her moods: it shone red when she was angry, blue when she was sad and green when she was happy.
![opal birthstone opal birthstone](https://a.1stdibscdn.com/1925-art-deco-black-opal-diamond-platinum-gemstone-ring-for-sale-picture-10/j_11843/j_165764821659387198485/_DSC3572_master.jpg)
It may even have been an invention of Sir Walter Scott in the novel Anne of Geierstein, published in 1831. This superstition probably is not rooted deep in history but only goes back to the early or middle 19th Century. There is a superstition that suggests that it is bad luck to wear an opal if opal is not your birthstone. These changes were accepted and approved by the American Gem Society and the National Retail Jewelers Council. Opal was originally the gem trade's recognized birthstone for October but in 1952 the Jewelry Industry Council suggested that pink tourmaline be an alternate stone to opal. If you were born in October, you are one of those people who has a choice for your birthstone.