Posted on October 20, 2016 by Mary Hood | 0 Comments
Opals consist of hydrated silica and occur in rock fissures. Due to their structure, opals are considered mineraloids rather than minerals. Opals are the national gemstone of Australia but are also found in Ethiopia and Virgin Valley, Nevada.
Opals have been the subject of various lore. During the Middle Ages, opals were associated with great luck and believed to carry the properties of every gem whose color matched one of the many colors reflected in the opal. Opals were also believed to bestow the power of invisibility. By wrapping an opal in a bay leaf and holding it in your hand, you could avoid being seen—or so the story goes.
Love of opals and belief in their inherent goodness dramatically changed with the publication of Sir Walter Scott’s Anne of Geierstein in 1829. The book describes a character who dies shortly after her opal comes into contact with a drop of holy water and turns black. Shortly after the book's publication, opal sales dropped by 50% in Europe and remained low for the next 20 years.
The Different Types of Opals
Black/Dark Opals are the rarest and most valuable opals. Found in the Lighting Ridge in New South Wales, black or dark opals have a naturally dark background, which allows their colors and rainbow tones to appear more vibrant. This natural layer of potch (colorless opal) on the back of the stone varies in darkness; the darker the potch, the more vibrant the colors in the stone. The more vibrant the stone, the greater its value. Most black/dark opals are cut into ovals or teardrops.
White Opals or “milk opals” are light with a white body tone (as opposed to the black/dark body tone of the black/dark opals). Mined in southern Australia in the opal fields of Coober, Pedy, Mintabie, and Andamooka, white opals are the most common opals and therefore the least valuable--but they can still be quite pretty.
Crystal Opals can be light or dark and are partially transparent. Partial transparency may enhance the color (and value) of a stone. An opal with transparency may be referred to as a “white crystal opal” or “black crystal opal” depending on its body tone.
Boulder Opals form in ironstone cavities in Queensland. They’re typically cut with some solid brown ironstone remaining on the back; the ironstone backing functions like the dark potch on black/dark opals, allowing the colors in the opal to stand out vibrantly. Found in Quilpie and Winton, boulder opals vary greatly in size and may be found as small as a pea or as large as a car. These are the second most valuable opals and are distinct for their thin, colorful veins. Boulder opals tend to have a flat or undulating surface and are almost always cut in a freedom shape, which maximizes the size the of the stone.
Matrix Opals occur as a network of veins between crevices in the host rock (usually claystone or ironstone). An andamooka matrix opal is a kind of matrix opal that has been enhanced by soaking in a sugar solution and boiled in acid, a process that deposits carbon in the stone’s pores, creating a darker background. A natural oulder opal matrix is a matrix opal in its natural state; it consists of brown ironstone with small deposits of opal.
Polished Yowan Nut Opal
Yowan Nuts are found in Yowan in Queensland. These ironstone concretions resemble nuts, which can be cracked open to reveal a valuable opal in the center.
Synthetic Opals are made in a lab with opaline silica, whose structure is similar to that of natural opal. Gilson Opals are the most well-known lab-created opals. Synthetic opals generally show brighter colors, are larger, and have a more ordered array of colors. Numerous subgrains in synthetic opals produce a delicate snakeskin pattern.
Imitation Opals are made with colored tinsel set in clear plastic or epoxy. They’re generally not convincing to the untrained eye.
Doublets and Triplets are partially man-made stones that imitate black opals. Doublets and triplets consist of a slice of opal attached to a dark backing. In addition to being attached to a dark backing, triplets have a clear quartz or glass capping to magnify and protect the stone while giving it a rounded appearance.