🍂 October Birthstone: Opal 📿
Good wishes to all and sundry whose birthdays fall in October! The Birthstone for October is Opal, a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl and basalt.
There are three broad classes of opal: precious, common and fire. Precious opal exhibits colour play known as iridescence, or more precisely, opalescence; whereas common opal and fire opal do not. Play-of-color is defined as “a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light.” The internal structure of precious opal causes it to diffract light, resulting in iridescent play of colour. Depending on the conditions under which it formed, opal may be transparent, translucent or opaque, and the background colour may be white, black or nearly any colour of the visual spectrum. Black opal is considered the rarest, whilst white, gray and green opals are the most common. On the whole, the degrees of opalescence and transparency are the major determinants of the classification and desirability of a piece of opal:
Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opal (hydrated silicon dioxide). However, there are three notable types of opal (precious, common, and fire), each with different optical effects, so the intended meaning varies depending on context. The optical effects seen in various types of opal are a result of refraction (precious and fire) or reflection (common) due to the layering, spacing, and size of the myriad microscopic silicon dioxide spheres and included water (or air) in its physical structure. When the size and spacing of the silica spheres are relatively small, refracted blue-green colors are prevalent; when relatively larger, refracted yellow-orange-red colors are seen; and when larger yet, reflection yields a milky-hazy sheen.
Precious Opal. The general definition of opalescent is a milky iridescence displayed by an opal which describes the visual effect of precious opal very well, and opalescence is commonly used in lay terms as a synonym for iridescence.
Common Opal. In contrast, common opal does not display an iridescence but often exhibits a hazy sheen of light from within the stone—the phenomenon that gemologists define strictly as opalescence. This milky sheen displayed by opal is a form of adularescence.
Fire Opal is a relatively transparent gemstone with a vivid yellow-orange-red color and rarely displays iridescence.
In a physical sense, some cases of opalescence could be related to a type of dichroism seen in highly dispersed systems with little opacity. Due to Rayleigh scattering, a transparent material appears yellowish-red in transmitted white light and blue in the scattered light perpendicular to the transmitted light. The phenomenon illustrated in the bottom photo is an example of the Tyndall effect.
Summing up opal as being “[m]ade of water and quartz, but filled with fire”, Arlene Goldberg-Gist describes the various myths associated with the gem in a 2003 article entitled “What’s that Stuff? Opal” published in Chemical & Engineering News as follows:
Greeks believed [that] opal bestowed its owner with the powers of foresight and prophesy. Romans perceived opal as a token of hope and purity. Arabs believed [that] it fell from heaven. Medieval peoples, however, associated opal with the Evil Eye and even the Black Plague or thought [that] it made a person invisible when the gem was wrapped in a bay leaf. Queen Victoria boosted opal’s popularity by making it a court favorite. More recently, as October’s birthstone, opal is thought to bring luck–but only to those born in October.”
Historical superstitions have also been elaborated by Wikipedia:
In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal. It was also said to grant invisibility if wrapped in a fresh bay leaf and held in the hand. As a result, the opal was seen as the patron gemstone for thieves during the medieval period. Following the publication of Sir Walter Scott‘s Anne of Geierstein in 1829, opal acquired a less auspicious reputation. In Scott’s novel, the Baroness of Arnheim wears an opal talisman with supernatural powers. When a drop of holy water falls on the talisman, the opal turns into a colorless stone and the Baroness dies soon thereafter. Due to the popularity of Scott’s novel, people began to associate opals with bad luck and death. Within a year of the publishing of Scott’s novel in April 1829, the sale of opals in Europe dropped by 50%, and remained low for the next 20 years or so.
Even as recently as the beginning of the 20th century, it was believed that when a Russian saw an opal among other goods offered for sale, he or she should not buy anything more, as the opal was believed to embody the evil eye.
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