Opal

   Opals stand in a class by themselves. More than any other gem, each opal is distinctly an individual. No other stone has as rich and varied folklore. They are both one of the luckiest and unluckiest gems a person can own. They are so unique, they have their own descriptive vocabulary. Opals are also the most delicate gems commonly worn. They require special care to insure their health and longevity.
 The name evolved from the Greek "Opallus" which means to see a change in color. Later, the Latin word "opalus" came to mean precious stone.
 There is some doubt that ancient authors were referring to the same stone we call opal today. Some scholars believe that many references are actually to iridescent gems, like iris agate.
 Opal's fire was long thought to be the result of iridescence. However, with the advent of scanning electron microscopes, we now know that it is a result of diffraction.
 Opal is an amorphous form of silica, (SiO2.nH2O) chemically similar to quartz, (SiO2) but containing 3% to 21% water within the mineral structure. Gem grade opals are usually 6% to 10% water content.
 Opal is a sedimentary stone. Under proper conditions, water percolates through the earth, becoming rich in dissolved silicates. When it enters a cavity, the silicates are deposited as tiny spheres. If they are uniform in size and shape, they will diffract light. If they are random in shape and arrangement, we have common opal.
 Volcanic ash gives black opal its color, but inclusions have nothing to do with the play of color. That is due entirely to the tiny spheres. They must be smaller than 1500 angstroms for blue and violet colors, but no larger than 3500 angstroms to produce oranges and reds. To put that in perspective, 20,000 spheres are about the size of the period at the end of this sentence.
 Opal grows by filling in cavities, regardless of their shape. Hence, we have many pseudomorphs, materials with shapes that are unrelated to the chemical content. The most common are opalized wood and seashells.

Black opal, precious opal with black body color. Also used for black potch covered with thin layer of crystal opal that lets the black under layer show through.

Semiblack, or gray opal, precious opal with dark body color.

Light opal is in between semi-black and white.

White opal, precious opal with white or very light body color.

 Crystal opal, transparent to semitransparent, colorless body with play of color.

 Doublets and Triplets make use of opal that is too thin to use as a solid gemstone. A doublet is a thin layer of precious opal glued to a black base. A triplet adds a transparent, quartz cap.
 Purists prefer the base material to be common opal. However many black materials are used, including old phonograph records.

Black crystal opal, transparent to semitransparent opal, with dark body color and play of color.

Fire opal, translucent to transparent, with yellow, orange, or red body color. May or may not have play of color. Also called Mexican opal or Sun opal.

 Boulder opal, a thin seam of precious opal on ironstone matrix. Since this is a natural occurrence, its value is higher than that of a man made doublet. The ironstone is very dark, which makes the fire stand out and gives a close resemblance to black opal.

Matrix opal consists of thin seams or spots of precious opal in matrix. The matrix is usually dark ironstone, but occasionally a light sand stone. Also called Mass opal.