The flashing colors and patterns of light in an opal are called its play-of-color or POC. For centuries the cause of this gemstone’s inner liveliness was a deep mystery and source of superstition. An opal’s play-of-color was proof of its magical powers and superior healing properties. Opals were said to be the tears of Zeus. They were slivers of shattered rainbows. They grew in places where lightning struck the ground.
Of all precious stones, it is opal that presents the greatest difficulties of description, it displaying at once the piercing fire of carbunculus [ruby], the purple brilliancy of amethystos [amethyst], and the sea-green of smaragdus [emerald], the whole blended together and refulgent with a brightness that is quite incredible.
~Pliny the Elder (AD first century)
The Natural History (Trans. J. Bostock)
There are two basic types of opal: common opal, which has no POC and little commercial value, and precious opal, which is the rare, shimmering variety coveted by collectors. For centuries scholars struggled to explain what caused the difference between these two types of opal. Many of the explanations focused on peculiarities in the structure of opals, which isn’t crystalline like the majority of other gemstones.
The cause of its…various delightfull colours, Cardanus saith is this, because the stone is crisp, and of some pervious parts: by those parts which are not pervious or porous it doth receive the light, and returneth it; and by this means it cometh to passe that it hath a white snowy brightnesse and splendor.
~Thomas Nicols (1652)
A Lapidary, Or The History of Pretious [sic] Stones With Cautions for the Undeceiving of All Those What Deal With Pretious [sic] Stones
Opal is a hydrated form of silica. Most opals are 3–10 percent water, but they can be up to 20 percent water. Because they always contain some water, it was once thought that POC was caused by reflections off water inside the stone. Others speculated that POC was caused by a series of parallel layers or laminae in the stone’s interior, which dispersed light like a prism. The French mineralogist René Just Haüy (1743–1822) believed that POC was caused by thin films of air that filled interior cavities.
Still others believed that Isaac Newton (1642–1727) was the first to determine the cause of opal’s play-of-color. According to Sir David Brewster (1751–1868), a Newton biographer and inventor of the kaleidoscope, Newton attributed POC to “undulating fissures of microscopic minuteness upon which refraction and decomposition of light takes place.”
The cause of opal’s play-of-color was not definitively explained until the 1960s, when two Australian scientists John V. Sanders and Peter J. Darragh peered into the interior of opals with the aid of an electron microscope. Under high magnification, they discovered that opal was made of tiny spheres of silica, which were closely packed in a three-dimensional grid or lattice. They further determined that the size and regularity of the spheres in the lattice framework made all the difference between precious and common opals.
In common opal, or potch, as it is sometimes called, the silica spheres are unequal in size and randomly arranged. In contrast, the structure of precious opal is regular and orderly. The silica spheres in precious opal are nearly identical in size (200–300 nm) and arranged in neat rows. The evenly spaced voids in the three-dimensional framework allow for the diffraction and interference of light that creates POC in precious opal.
We also know that minute differences in the size of the silica spheres (within the 200-300 nm range) are what determine the color of the light that is seen by the naked eye. Red, an unusual and valuable color in opals, occurs when the spheres (and their associated voids) are relatively large (~300 nm and larger). Blues and greens result when the spheres and their associated voids are smaller, a more common occurrence, which is why those colors are more often observed in precious opals.
The opal, when pure and uncut in its native rock, presents the most lovely colors seen in the world, except those of clouds…
19th-Century Art Critic
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