Play of Color
Phenomenal gemstones are those that interact with light in unusual ways. This may be expressed as a cat’s eye or a star within the jewel, color change, or a unique sheen. Opals may exhibit play of color, the result of opal’s silica spheres acting as prisms, turning white light into an array of colors. Vivid play of color is rarer and more likely to catch the eye than play of color that is moderate.
Play of color is evaluated in different ways. One classification is color range, or the number of hues which appear in the opal. The types of color also matter, with some hues being rarer and more in demand than others. Some opals may display just one color, or have two or three hues play across its background. A small number of opals display every color of the rainbow.
Hues are determined by the size of the silica spheres. At their smallest, 0.1 microns, the silica bends light in a way that produces purple. The largest size, 0.2 microns, creates red. Diameters in between these measurements create other colors. Blue is the most common hue, followed by green. Red is the rarest color; opals with large amounts of red can fetch high prices.
How the play of color appears on the jewel is known as a pattern. Opal patterns can be organized into hundreds of different categories, often with fanciful names like “church window” or “peacock.” The three basic terms are “pinfire,” “flash,” and “harlequin.” Pinfire is play of color that is made of a series of tiny sparkles. The amount of pinfire on a stone may range from a few flecks to great clusters of dots.
Flashes are larger shows of play of color on an opal with irregular edges. They may look like small squiggles on the stone, or take up most of the jewel. Harlequin patterns feature rectangular patches of play of color whose edges touch one another. They may also be described as mosaics.
Potch Opal
Play of color happens when the pieces of silica in an opal sit neatly against one another in a grid pattern. When spheres are arranged willy-nilly, optical phenomenon doesn’t occur. These types of stones are known as common or potch opal. Scientists believe that when the mixture that becomes opal dries quickly, the silica arrangement become chaotic. Then the mixture dries slowly, silica is more likely to form an orderly pattern.
Common opal may lack play of color, but have their own beauty. They come in many colors, such as brown, white, blue and pink. Some potch opal possess clarity characteristics, which in turn give the gemstones names like agate opal for banded jewels, moss opal for vine like tendrils, or jasper opal for when jasper and opal merge as one.
Certain types of potch opal are capable of exhibiting optical phenomena. Derived from the Latin words for “turn” and “sun,” “girasol” is used to describe gemstones whose sheen moves with the jewel as it’s moved about. Girasol opals are bluish gems that displays reddish reflections. They’re translucent enough to be faceted, which provides more surface area to showcase its reds.
Contra Luz
Some opals have a transformation act. Under most circumstances, they resemble common opals. When held up just so, light travels through the gemstone to the eye, displaying play of color. Such gems are known as contra luz opals, after the Spanish words for “against light.” They don’t reflect light back to the viewer, but needs light to travel through them in order to look their best.
White Opal
White opal has a white to medium grey background color, or hue that isn’t influenced by play-of-color. This jewel is available in the commercial, middle and high end markets, meaning that there is a white opal for every budget. White opal has also been mined for much longer than many other forms of opal, making it the type that most people are familiar with.
The quality of white opal can vary greatly, from a dominantly white stone with subtle play of color to jewels with bright overall patterning. When selecting a white opal, look for one that has few cloudy patches. Its surface should be free of pits, fractures or other blemishes. White opal should also possess little to no host rock.
When placed against a dark background, white opal’s play of color may stand out more strongly. This in turn allows wearers to play with the appearance of their opals. Placing opal jewelry over light colored clothing will show the jewel’s regular intensity. The same piece over dark clothing may make the play of color seem more vivid.
Black Opal
Black opals are known for their dark background colors. Despite the name, these colors may be deep blue, grey or green as well as black. If it looks black in reflected light, it’s considered a black opal. The dark coloring helps the play of color stand out in sharp relief, catching the eye. The darker the background, the more intense the play of color. Black opals can be transparent, translucent or opaque.
There are multiple subcategories of black opal. Black crystal opal is transparent to semitransparent, and is admired for its vivid play of color. Semi-black opals have grayish backgrounds and can also be translucent, as well as semitransparent and opaque. The finest semi-black opals appear semi translucent when placed against the light. Grey-base black opals too have grey backgrounds, but appear opaque even when held to a light source.
For many years, black opals were a hidden gem, so to speak. They were discovered in 1902 in New South Wales, Australia. This heralded a boom in Australian opal mining, with sources found in several provinces. Black opals were also discovered in Nevada around 1905, with the first mine established in 1908. Gemstone quality black opals continue to be mined in these areas. Black opal isn’t as well-known in many parts of the world as its white counterpart. It’s relatively unknown in parts of Europe and the United States, though the US and Japan have a fondness for black opal. Black opal can command the highest prices of all opal types. This is partly due to rarity, as few places in the world provide the gemstone. Demand also serves as a factor, since black opals’ play of color can look so dramatic.
Crystal Opal
Crystal opals are so named due to their transparency, with semitransparent versions known as semi-crystal opals. Their background colors can be white, yellow, blue, black or colorless. When backlit, these opals resemble gelatin, earning the nickname of jelly opal. This can be confusing, as “jelly” is also a nickname for water opals.
One of the other distinctive features of crystal opals are their play of color. The optical phenomenon is unusually strong in this type of jewel, leading to colors with powerful saturation. Translucence adds to the beauty of the stone, as play of color is not only viewed on its surface, but also inside the gem. As with most opals, crystal opals are usually shaped into cabochons, though some versions may be fashioned into faceted cuts.
Water Opal
Water opal is often confused with crystal opal, despite their distinct characteristics. While crystal opals show vivid gem phenomena, water opals have little to no play of color. They get their name from their high water content, which can be as great as 21%. Water opals are also noted for their gelatinous appearance and transparency, which makes them enough to facet.
Gemstone quality water opals are usually mined in Australia or the United States. They tend to be blue or blue grey, though one specimen from Hidalgo, Mexico is more orange brown, closer in hue to a fire opal. The stone was photographed still attached to its host rock. Even though a quarter of the stone was covered in water opal, the rock underneath was still visible.
Boulder Opal
The name “boulder opal” is slightly misleading. Boulder opals aren’t of massive size, but are found in veins containing ironstone, an iron oxide rich sandstone. When cut, the ironstone is left on the gemstone. Depending on the rough, the result may be opal with a visible ironstone matrix, or have a top side made of opal, with the ironstone covering the back of the gem.
Ironstone backing adds strength to a gemstone known for its fragility. The darkness of the ironstone against the opal also deepens the jewel’s background color, which in turn highlights the play of color. Depending on the cut, the darker color and pronounced optical phenomenon can make boulder opal resemble a black opal. A person on a budget who wants a gemstone with the appearance of a black opal may want to consider a boulder opal. Other types of boulder opals, with ironstone visible when viewed face up, are also used in jewelry design. The combination of opal and ironstone in these cuts can create a striped or patchwork effect. These jewels, also known as boulder matrix opal, are evaluated by their play of color and how attractive the two stones look together.
No two boulder opals are alike. Each stone is cut so the opal and ironstone aesthetically balance one another, and to bring out play of color. As the patterning varies with each piece of rough, the gemstones must be considered on a case by case basis. Most jewelry incorporating boulder opals have specially made settings in order to accommodate their non-standard sizes.
Fire Opal
Fire opals are considered a subgroup distinct from precious and common opals, with a different emphasis on quality factors. They are noted for their bright background colors, usually yellow, red or orange. These colors may be a result of trace iron. Some fire opals have play of color, but others do not. Even those without optical phenomena are valued for their background color.
Preferred fire opal has a uniform color throughout the stone. Orange-red and red tend to be more sought out than other hues. The more vivid the saturation, the more desirable the stone. Fire opals without play of color that are red-orange to bright red are known as cherry opals. Fire opals vary in transparency from opaque to clear enough to facet. Those that display play of color are usually fashioned into cabochons. Faceted opals are cut into a variety of shapes, such as round, square, pear and so on. One notable example, cut by Allyce Kosnar, is a round cut with a specially faceted pavilion. When viewed from the top down, carefully polished facets reveal the outline of a spider.
Other opal types may overlap with fire opals. For some, they exhibit play of color when held up against the light, making them contra luz gemstones. Others may be exceptionally clear, as with crystal opals. Still more might have an unusually high water content, giving them a gelatinous look like water opals.
Martian Opal
On October 2008, NASA reported several findings from their Mars Reconnaissance Orbiter. Not only were there signs that the planet’s surface once held water, there was a broad spectrum of minerals that scientists hadn’t known were there. When combined with volcanic and other natural activity, some of these minerals combined to form opals.
In some locations, such as the Valles Marineris canyon system, opal consists of thin sheets on outcrops, along the canyon rim and inside the system. Other places include Gusev Crater and old river beds. There are some guesses that the water that created the opals was acidic, but it’s uncertain if this was always the case. NASA is using the presence of opals in these locations as guides of where to search for water on Mars.
Opal Cuts
When it comes to cutting an opal, one crucial matter to consider is the jewel’s play of color. If the opal has optical phenomenon, the cutter must find a way to display it to its best advantage. Ideal cabochon cuts should be symmetrical, with a rounded surface to highlight phenomenon. However, top quality opals are usually fashioned into irregular shapes in order to preserve rough and show more surface area to display play of color.
Translucent or transparent opals, like fire or crystal, may be faceted. This helps to draw out their background colors as well as any internal play of color. Boulder opals get special consideration. Before they’re cut, they’re evaluated to see if they’re suited for a cut with an ironstone backing, or shaped to reveal the contrast between the matrix and the opal. If the latter is chosen, cutters try to find a pleasing balance of ironstone and opal. The more opal that shows, the more desirable.
