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OPAL (SiO2•nH2O): Rainbows in a Rock

Not a mineral, not a rock, opal is a geologic entity quite unique among mineraloids: it can contain up to 21% weight percent water. As such, it is possibly more correct to consider it as a frozen gel, but even that is confusing and inaccurate.

Rather than a crystalline structure (essential to the definition of what is a mineral), opal is made of an accumulation of silica spheres, each spherule on the order of 150 – 300 nanometers in diameter (one nanometer is a billionth of a meter): obviously it would take zillions of spheres to make opal in any appreciable amount. These teensy-tiny silica spheres settle together, rather like a pile of marbles in a box, with water occupying the space between the spheres. As coincidence goes, these sizes are the same as the wavelengths of visible light; light diffracting through the silica spherules and water creates rainbows of colors that change with the angle we view them. If the spherules are on the smaller size, then shorter visible wavelengths (towards blue) are diffracted, and if larger, then longer wavelengths (greens and yellows and reds) emerge. If the spherule sizes are too large or too heterogeneous, then light is not diffracted and the opal will not possess the radiant splay of colors that make it a valuable gemstone.

Pure opal is colorless. Impurities within the opal give a background color to the mineraloid. Iron, manganese oxides and organic carbon give rise to the creamy, orangey, and black colors of some opals. Milky opal includes small air bubbles.

Opal can form in many geologic environments:
—It requires a source of silica such as an igneous or volcanic rock, but nearly any kind of silicate rock will do in a pinch;
—The silica spheres are produced via rock-water interaction processes. The presence of warm water aids these processes but no great heat is essential. Acid conditions, the presence of clays as an intermediate phase, and even bacterial activity have been proposed as requisite for the formation of the silica spherules.
—Silica sphere-bearing waters then flow into small open cavities such as cracks in a rock, vesicles in lava, or the pores of a fossilizing tree;
—There, the small spherules settle out of the waters, accumulating as layers within these open voids.
—As they accumulate, the size of the spherules can change, adjusting amongst themselves to create uniform sizes with the potential of producing a gem-quality stone.

It has been calculated that to form a one-centimeter deposit of opal at a depth of 40 meters in the earth can take 5 million years!

Annie R

Photo by Tobias Schorr courtesy . In situ opal from Gashena, Ethiopia.

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