October Birthstone Opal Mohs scale hardness: 5.5-6.5 The name opal is actually means “to see a change in color”. These Australias national gemstones are also given to commemorate a 14th wedding anniversary. The word “opalescense” was coined to describe opal’s iridescent play of colors. Opal is a hydrated amorphous silica that refracts light and reflects it in a play of colors. The stone comes in a vast range of patterns and vivid color combinations, making it the most dramatically varied of gemstones. One of the primary ingridients in opal is water (between 3 and 10%, but can be as high as 20%), so care should be taken not to let it dry out or it could be susceptible to cracking. Opals must also be guarded from blows, since it is quite soft and break easily. How Opal is Formed Opals formed at low temperatures from silica-bearing waters and they can occur in fissures and cavities of any rock type, usually sedimentary rock. They are formed when the sediment is laid down, pressed and buried to make rock. Water carries silica into the cavities and it is left behind when water evaporates. Silica collects into spheres of uniform distribution and size, they packed together efficiently thus creates precious opal. Sometimes fossils become opalised, which is the formation takes the place of a bone, teeth, shell or other organic matter, preservng a record of the object as it decomposes away. Colors of Opal Opals are classified according to their body color and the play of color they have. The color of the gem is due to fine-grained impurities. Like agates, every opal is unique. Precious opals interact with light to create flashes of the colors blue, yellow, green, red and orange, plus aqua and purple, occasionally. Not every opals show these colors. The most common is blue flashes, then blue occuring with green, then blue occuring with yellow and green, then those plus orange, with red flashes the most unncommon. The most popular opals are orange and red opals (also known as fire opals). Opal which displays red can show other colors too. The body color of the stone can be milky (white opal), clear (water opal), gray, brown or even black. In fact, black opals command the highest prices. Opal Play of Color Precious opals have a rich iridescence and remarkable play of changing colors. This is due to the regularly packed uniform spheres of amorrphous silica a few tenths of a micron in diameter in the internal structure of the stone. The colors displayed is determined by the sphere diameter and the refractive index. Spheres packed together leave gaps. Light passes and bounces around through the spheres and the gaps among them. It enters some of the material and bends then diffracted, rainbow colors appear when light split. When light comes back out of the opal, what you see is the colors the opal and light have created. When the stone is moved, light hits these spheres from another different angles and bring about a change in color. The largest spheres produce the red colors, while the smallest ones produce blue. They must be smaller than 1400 angstroms for violet and blue color, but no larger than 3500 angstroms to produce reds and oranges. The more uniform the spheres are placed, the more brilliant, intense and defined the color will be. It is the orderliness of these spheres which separates common opal (opal which do not possess the play of light) from precious opal. If they are random in shape and arrangement, what we have is a common opal. The way in which colors change witthin a particular stone as it is tilted and rotated is called the stone’s play of color. Common opal is also known as potch. Types of Opals There are 4 basic types of precious opals: white opals (also known as light opals), fire opals, water opals and black opals. White opals are the most common types, with a body color of white, cream or yellow. Fire opals are translucent or transparent, with no color play. They have honey-yellow to hyacinth-red and shows intense orange and red fire-like reflections. This variety is desired by many people due to the warm red body color they have. The stones have color throughout. It is the only variety that can be faceted. Water opals ,this variety has body color of transparent to translucent. It shows litle or no play of collor and hence is low in value. Black opals are the most precious type of opals, they have body color of black or dark gray (do note that black opal should not be completely black otherwise it would be completely worthless, it should displays a myriad of rainbow colors, the more colorful the better), sometimes dark green or blue are also called black opals. Top quality black opals can be more expensive and valuable per carat than diamonds. Crystal opals are also transparent, but they do have the play of color. Boulder opals (another type of opal) have a layer of stone from mining left on opal that occur in a thin layer. Boulder opals can command prices second only to black opals. Composite Opal - Opal Doublet and Opal Triplet Opals used in jewelry are usually one piece of solid opal. However, some opals are found in very thin layers (therefore is very fragile and easily break). To prevent that happens, the opal is sometimes glued to a base of obsidian, potch, basalt etc (usually black backing piece, thus resembling natural black opal). These two stones are called “opal doublets”. In order to protect the opal layer of the doublet, a transparent top is sometimes placed over it (often of quartz). Now the stone is made up of three pieces and rather look like a sandwich. What you have now is a “opal triplet”. A doublet opal is more valuable and expensive than a triplet because it has greater opal content. However, doublet and triplet opals can be damaged by prolonged exposure to water (or any liquid) as the water penetration can cause the glue to deteriorate and eventually separate the layers, causing the opal to take on a foggy or cloudy appearance. The origin of the “never get your opal wet” misconception comes from this fact. Australia is the world’s main producer of opal, although much of it is processed and traded in Asia. The majority of light opal, the most common and least expensive variety, is cut and polished in factories in China, then set into mass-market jewelry for distribution throughout the world. Japan is the primary market for the much more expensive black opal. Recently, Indonesia discovered several opal deposits on the island of Java. Only small quantities have been mined to date and most of it is set into souvenir jewelry sold within Indonesia. Among of all the hues in opals, reds against black are the most rare, while white and greens are the most common. Precious opals are multi-colored and known for their play of color. Darker body tones (main background color) are more valuable than light or white body tones. Opals are valued by many cultures, and could be found in both the new world and the old.
Posted on: Thu, 02 Oct 2014 20:27:07 +0000
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