Tips: Selecting Opals

Most people enjoy the beautiful colors, sparkle and fiery colors found in many opals from across the globe.

The most expensive opals are typically Black Opals found in Australia. These are known for their dark gray to black color tones, caused by the presence of carbon and iron oxides. Opals are also found in Hungary, Mexico and several western states in the USA.

Tip: Many opals on the market today are partially man-made. These are referred to as doublets and triplets. Slices of real opal are glued to a black backing in order to increase the vibrancy of the opals’ colors. Doublets are normally two slices glued together: the opal slice glued onto a black backing. The surface of the opal is often rounded to give it a domed shape. In triplets a very thin slice of opal is glued to the backing and an additional layer of quartz or plastic is added to create a surface layer which both protects the thin slice of opal and may serve to enhance the colors slightly.

If a doublet is glued into a piece of jewelry and the sides have been covered, it can be very difficult to tell whether or not the opal is real or a doublet.

Tip: Solid opals are usually fine when wet, however, doublets and triplets may get infiltrated by the moisture and can develop a gray or foggy appearance. Natural opal is brittle, heat sensitive, and may scratch or break easily. Opal has a hardness of 5 to 6 on the Mohs scale.

Opal Cabochons Australia

Fire Opal

Rough Opal

The term opal is from the Sanskrit word upala meaning “precious stone.” Opals are precious gemstones comprised of hydrated silicon dioxide. Over 90% of the world’s gemstone opals originate from Australia. The play of light (diffraction of light) on the tiny micro-spherical structures in opals, create the wide range of colors seen in opals. Opals do not have atoms in regular crystalline arrangements, but rather have tiny spheres which are aligned in planes.

Opals can range in color from light to dark, from transparent to opaque and can occur with or without the Play of Color, Fire, or play of light for which it is so prized. Opal is called Precious when it exhibits the Play of Color; and is referred to as Common Opal or Potch when it does not have the play of light. Potch has a less orderly structure at the micro-spheroid level and therefore has less play of light and color.

Opals are formed when water (hydrothermal solutions) mixes with silicon dioxides in a host rock, often in sedimentary rocks and also found in volcanic areas. As the water evaporates over one to two million years, the remaining amorphouos, gel-like silica material forms into tiny spheroids which are naturally arranged in a regular pattern, with tiny gaps that are also regular in structure, and which together allow a wide spectrum of light diffraction.

Boulder Opal refers to opal that remains attached to it’s host rock and Matrix Opal refers to opal that occurs within the pores and holes of the matrix of the host rock. Mosaic and Chip Opals are comprised of small pieces of natural opal held together in a resin matrix. Whereas synthetic opal is created in a laboratory and Imitation opal resembles the play of light found in natural opals but does not have any of the natural chemical structure of real opal.

For more information, visit http://www.gemstone.org/gem-by-gem/english/opal.html.

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Gem photo credits: Most of the photos of gemstones and minerals shown here are reproduced with kind permission of Roger Weller, Professor of Geology at Cochise College, AZ. Mr. Weller has many more beautiful photos of gemstones, minerals and rock examples. The Cochise College science and geology website is an excellent source of information for those that would like to learn more about the wonderful treasures and geologic evolution of the planet we live on.