A Midsummer Night's Gleam

Though "gleam" is hardly a proper term used in gemology, it conveys the idea of optical phenomenons which is at the core of today's entry. In gemology, we are often called upon to document and correctly assess the optical phenomenons which some gems display and to explain why they occur. While many of them are pretty cool when you look at the actual science of it, none are quite as spectacular, nor as seemingly random as the play-of-color in opal. Even pictures fail to adequately illustrate how special play-of-color can really be, so we hope that these samples will give a sense for it. Let's dive into it shall we?

The "Virgin Rainbow" opal, soon to be displayed at the South Australia Museum. Photo courtesy of www.atlasobscura.com

Unlike most gemstones, that have a crystalline system which dictates the way that it will grow and take shape, opal does not grow from any set system. However, it is made up of microscopic silica spheres that are tightly pact together; so it can't be considered completely random. Opals in sedimentary deposits occur when mineral-rich waters are rushed into cavities and fissures of rocks; the low temperatures over long periods of time will allow it to take shape. They can also be the product of a volcanic environment (see the recent Gem-A article on hyalite for more info).

Ethiopian Welo opal. Photo courtesy of www.opalauctions.com

Today, opal is found in numerous places (including Mexico and the U.S) though it is most abundantly found in Australia where they've been uncovering deposits since the late 1800's. It is from these deposits that we see some of the most beautiful and rare specimens, which display all of the colors of the rainbow. Recently, the emergence of Ethiopian opals has brought to light an altogether different looking opal, which is rapidly growing in popularity. We'll talk about their particular properties in a separate entry. Here are a few other varieties to consider:

Black opal. Photo courtesy of www.opals-on-black.com

• Black opals are generally the most sought after opals, as they tend to best display the play-of-color in the stone. The term "black" does not refer to the actual body color of the opal, but rather of it's darker background. When looking at opal doublets (which we'll cover in a different entry) for instance, you'll notice that they are backed with a darker material in order to enhance the color.

White opal. Photo courtesy of www.crystalsrocksandgems.com

• White opal, as with black opal, is named that way to describe the background of the stone. It is a much lighter looking stone; though the play-of-color is more subtle, it has a beautiful whimsical quality.

Water opal. Photo courtesy of www.cynthiarenee.com

• Water "Jelly" opal has always been pleasing to me. As a relatively colorless stone, the play-of- color looks like bits of color trapped in a chunk of jelly which is all too comical.

"Boulder" opal. Photo courtesy of www.treasurion.com

• "Boulder" opal refers to the opal that is often too thin to be cut away from the host rock that it was formed on. In some cases, the boulder's contrast to the colorful opal is rather pleasing and makes for an interesting gem on its own.

"Opalized" wood. Photo courtesy of www.mymodernmet.com

• "Opalized" materials are a really cool concept. As mentioned earlier, opal is made up of silica spheres. In the same way that petrified wood is the result of silica taking over the internal structure of the wood, opalized material is the result of silica invading its internal structure of those materials, giving them the appearance of opal. This can be seen in fossil and wood alike.

So where does the color come in, you may ask? Well that's also a cool bit of science:

 Illustrates the microscopic interaction between light and opal's silica spheres called diffraction. Photo courtesy of www.olympicopals.com.au

As we've discussed in an earlier entry, light is not white at all, but rather a whole spectrum of colors. But when light reaches the microscopic spaces between the silica spheres in opal, light is made to diffract or "split" into all of the spectral colors. The diffraction of these colors on all of these spheres subsequently creates interference with one another as well. This is why the color is not static in the stone, but rather moves around when you move around the stone. The colors themselves are dictated by the size and compactness of the silica spheres, which is why we see such a range in colors.

Of the opals pictured in this entry, which do you like best? Let us know!