Lightning Strikes Once!

Opal,  since early Roman times,  was regarded as one of the most treasured gems until the 19th century,  when Sir Walter Scott's novel "Ann of Geierstein" associated this fiery stone with an unfortunate heroine.  The story's heroine,  accused of being a demon,  died shortly after her opal lost all its color when sprinkled with holy water.  As the book became more popular,  superstition also increased and opal became associated with bad luck. But this was just superstition and not fact!  Queen Victoria helped dispel the curse on opal by giving opal jewelry to relatives as wedding gifts.  Also,  in 1877 a remarkable find of Black Opal in an area of New South Wales (Australia) called Lightning Ridge,  revived the opal market.  These Australian opal literally took the world by storm!  Even today,  there are no other sources producing opals as fine as those from Lightning Ridge. Since Black Opal is far rarer than White Opal,  it commands much higher prices.  These two varieties of opal (white and black) are found in different rock formations;  Black opal is typically found in nodules,  whereas White Opal is found in seams.  Two mines in Australia,  "Coober Pedy"  and  "Andamooka"  account for 75% of the world's supply of white opal. But what makes an opal black or white?  Black Opal is so called by virtue of its dark background color - black, gray, blue or green - with vivid flashes of pure spectral hues.  White Opal has a light background with flashes of colors that are more pastel. There are some basics to opal beauty.  First and foremost is the base color, also known as the  "potch".  The darker the potch,  the more intense the play of color,  a direct result of contrast between the color play and background.  A second factor to consider is Hue Mixture.  The purity,  distribution,  and number of different hues displayed in an opal greatly affect value.  Naturally,  the more varied and pure the colors,  the more lively the stone and hence the more valuable. Finally,  connoisseurs consider Color Pattern.  One of the most favored patterns,  called Harlequin or Mosaic,  features well-defined geometric patches of color in both symmetrical and asymmetrical designs.  Other distinguished patterns include Flash Opal which displays broad well-defined flashes of color that change as the stone moves,  much like the prisms seen in a turning Kaleidoscope.  If the colors in a Flash Opal are predominantly red,  orange and yellow,  then the stone is referred to as a Flame Opal. Pinfire Opal is a term used to describe stones with pointillistic dots of color.  As the dots get smaller and less vivid,  the value decreases.  Peacock Opal is a name reserved for opals displaying predominantly blue,  green and violet flashes of color.  What causes these magnificent patterns of color in opal?

Opal is composed primarily of silica and some water molecules.  The phenomenon Play-of-Color is the result of light diffraction on these silica and water spheres.  The size and number of these molecules as well as the ratio of silica to water determine the intensity,  number and distribution of colors portrayed.