Rear view of the opalized Addyman fossilized plesiosaur at the South Australian Museum. Opalized wood is also called xylopal or zeasite. These opals are called pseudomorphs, materials with shapes unrelated to their chemical content. In effect, opal replaces these organic materials. Since opals grow by filling in cavities in the earth, sometimes they take the shapes of pieces of wood, bone, and seashells buried in the ground. While it's acceptable to refer to an opal's play of color as fire, it's not acceptable to refer to gemstone dispersion as 'play of fire.' Only opals show play of color. Please note that the term 'fire' typically refers to a gemstone's dispersion. (An angstrom is one ten-billionth of a meter). To put that in perspective, 20,000 spheres are about the size of the period at the end of this sentence. They must be smaller than 1,500 angstroms (Å) for blue and violet colors, but no larger than 3,500 Å to produce oranges and reds. That is due entirely to the tiny silicate spheres. For example, black opal gets its color from volcanic ash, but inclusions have nothing to do with the play of color. The particular colors seen in an opal's fire depend on the size of the spheres and the angle of viewing. This phenomenon of flashing or moving colors due to diffraction isn't related to the body color of the opal. However, with the advent of scanning electron microscopes, we now know it's a result of diffraction. Opal's characteristic fire, or play of colors, was long thought to be the result of iridescence. Precious opals, also known as 'noble opals,' display fire or play of colors. Seldom cut, these stones come in a wide range of colors. If the spheres are random in size, shape, and arrangement, the results are common opals.Ĭommon opals can have an opaque or glassy appearance with a waxy luster. If the spheres are uniform in size and shape and neatly stacked, they will diffract light. When water enters a cavity, it deposits the silicates as microscopic spheres, forming opals. Under the proper conditions, water percolates through the earth, becoming rich in dissolved silicates. Gem grade opals usually have 6% to 10% water content. Opal is an amorphous form of silica, chemically similar to quartz, but containing 3% to 21% water within its mineral structure. What's the Difference Between 'Common Opals' and 'Precious Opals'? Black opal, Australia (stone in bracelet ~ 20 carats). (So rich, in fact, opal gems have been considered both the luckiest and unluckiest stones you can wear). Nevertheless, some scholars believe many ancient references to opal may actually have been to other gems, such as the iridescent iris agate. These striking gems have inspired a rich body of folklore.

Opal Stone Facts and Informationįor thousands of years, people have mined and treasured opals.

Gold, enamel, opal, pearl, and diamonds, 7.6 x 6 cm. In this pendant by René-Jules Lalique, two enamel peacocks stand on a triangular opal cabochon. Nature scenes and animals were popular motifs in Art Nouveau jewelry, and opal gems were popular jewelry stones during this period. 78 opal cabochons from Welo, Ethiopia (52 ctw) in 18k gold bracelet. For details, see our articles on Appraising Opals and our Opal Buying Guide.

The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.Įvaluating precious opal is a highly refined art.

Types of opal