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Opals were once believed to have supernatural origins, falling to the Earth in a burst of lightning. It isn't hard to understand why, as their rainbow-esque color is positively magical. Every opal is a one-of-a-kind stone with varying play-of-color. The name opal is derived from the Latin opalus, which means "to see a change of color." There are two kinds of opal; only one displays the various colors – precious opal.
Many gemstones are graded on three facets of color – hue, saturation, and tone. Tone refers to how much light is absorbed or refracted, hue is how dark or pale the stone is, and saturation is how much of the stone is the valued primary color.
However, opals tend to fall into their own category due to their play-of-color. Play-of-color refers to how many different colors of the spectrum are visible in an opal. It occurs due to their composition. Opals are made up of silica spheres and trapped liquid. The silica spheres don't have other gems' organized crystal lattice structure. Still, when the spheres are arranged into a symmetrical pattern that repeats, magic happens. Light enters the stone and has to bend to squeeze through the gaps between spheres. As it bends, the light is split into all spectrum colors.
Opals with disorganized spheres are known as common opals. They are not anywhere near as valuable because they cannot break light into all the spectrum colors, meaning they lack the gorgeous play-of-color seen in precious opals.
The play-of-color can vary from opal to opal due to the size of the gaps between silica spheres. Small gaps display greens and blues, while large gaps produce reds, oranges, and yellows. Reds as the dominant color in the play-of-color are pretty rare, followed by orange. These colors are the most valuable.
Blue and green play-of-color are more common but can still be very pretty. The colors can also be displayed in various patterns, including peacock tail, Chinese writing, flagstone, jigsaw, and stripes, as well as many others.
In addition to the dominant color in the multi-colored display, it's essential to consider how much the color is spread across the stone's surface. Any patches where the play-of-color is not visible are considered the body tone.
An opal's body tone simply means how dark the background is or the portion of the stone seen behind or around the play-of-color. The body tone can range from dark (black) to very light (transparent). Grades are assigned to body tone beginning at N1 (jet black) and ending at N9 (transparent crystal).
Black opals are the most valuable and range from N1 to N4. Semi-black opals comprise N5 to N6, and white opal crystals are assigned N7 to N9. The stone must always be viewed face up with a disregard for the play of color when assessing body tone.
The opal body can also have a dominant color in addition to its level of darkness or lightness. Opals with a black body color are often very sought after, with specimens designated "black opal" very rare. However, you can also find opals with a green, blue, purple, white, orange, or colorless body.
Crystal clear opals with extremely bright reds, oranges, and yellows as their background color are known as fire opals, one of the most prized types of opals. Ethiopian opals are colorless but possess high play-of-color; this type is relatively common. Other opal types based on body color include Peruvian or blue opal and honey (yellow) opal.
Interestingly enough, opals from different geographical areas tend to have different colors. South Australia commonly produces white or milky opals, while New South Wales is where the elusive black opals are found. Ethiopia and Kenya typically are sources of white opals though fire opals can also be found there. Peruvian opals are typically pastel-colored white Brazilian opal has a crystal body with an almost neon play of color.
Brightness is determined by how much light is bounced back to the eye by the gemstone. It is also known as brilliance. Opal brightness refers to the surface brilliance and the play-of-color inside the stone. If the play-of-color is dull, the opal will not be as valuable. Sometimes this can happen because pieces of the rock in which the opal formed become trapped inside the gem. There are occurrences when this can still be prized, such as with Boulder or matrix opals, but the play-of-color must still be brilliant.
Opal transparency only becomes a factor in opals with a lighter color or tone. Crystal opals or white opals are more valuable if they are transparent. This is due to the play-of-color being visible deeper into the stone if it has good transparency. The most valuable of the transparent category is water opal which is almost entirely transparent and colorless.
Opals are sometimes treated, though this usually makes them less valuable. Various heat treatments can be applied to try and enhance their play of color. Some treatments involved soaking the stone in a sugar solution and then sulphuric acid. The sugar is carbonized and burned to blacken the body color of the opal and contrast more with the play of color.
Treatments with oils, plastics, resins, and wax can be used to try and fill gaps or remedy crazing. Sometimes smoking the opal by wrapping it in paper and heating it with smoking materials is used so that black soot particles permeate the stone and darken the surface.
In general, opals are more valuable if they are not treated to enhance color or durability.
Opal can come in a variety of shades. The body color and tone can range from black to white, as well as almost every color in between. The play-of-color is often a rainbow but can have a dominant shade.
Black opals, referring to their body color and tone, tend to be very valuable. Fire opals, with a red or orange body and play-of-color, are highly sought after too.
Opal is a very precious gem and quite valuable depending on its color. Because each one is unique, there is really no substitute for an opal.
Opals tend to be one of the most popular stones for an engagement ring. However, it only ranges from 5-6.5 on the Mohs scale, making it easy to scratch. They're also susceptible to water, oils, and lotions due to their high porosity.