All About Opal

It is late and I just turned my soldering iron off. I am giving it my best shot to make as much mineral art as I can for the two upcoming shows. It is hard to just go to sleep right away after working such a long day, so I decided that I'll do a blog update now rather than wait until morning.

This update is about opal. Opal is a non-crystallized form of silica that contains between 3% and 21% of  total weight is water, but usually has only 6% to 10% water. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock. It is most typically found with limonite, sandstone, rhyolite, marl and basalt. Opal is the national gemstone of Australia, which produces 97% of the world's supply.

Opal's internal structure makes it diffract light. Depending on the conditions in which it formed it can take on many colors ranging from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. It varies in optical density from opaque to semi-transparent. For gemstone use, its natural color is often enhanced by placing thin layers of opal on a darker underlying stone, like basalt. 


Even though opal is not a crystallized form of silica, it does have an internal structure. At micro scales precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. It is the regularity of the sizes and the packing of the spheres in opal's structure that determines the quality of precious opal.

Opal's structure is depicted in the diagram below.

In the source rock, veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of opal, backed by a swart mineral such as ironstone, basalt, or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter base material.

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974. The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Furthermore, synthetic opals do not fluoresce under UV light.

In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal. Opal is considered the birthstone for people born in October or under the sign of Scorpio and Libra.