Opal - an amorphous composition of microspheres of cristobalite, SiO2.nH2O, precipitated from silica gel into layers or nodules in veins and cavities of volcanic and sedimentary rocks in numerous areas of the earth. Water is always present in opal, but the amount varies considerably. This accounts for the frequent practice of storing opal in water or oil. The diaphaneity of opal varies from transparent to opaque.
True Opal - refers to the characteristic of an uninterrupted display of fine colors when viewed from any surface angle. It does not refer in any way to the chemical composition or other properties of opal. All precious opal has this quality.
Play of Color - the result of the regular arrangement of remarkably uniform sized microspheres of silica with its corresponding regular array of the tiny three dimensional "holes" that form a diffraction grating for visible light. At various angles, the different wave-lengths of diffracted incident white light, when split into its spectral colors, are reinforced by each other, and we observe the play of color. With the exception of the Contra Luz variety of opal found in the volcanic areas of Australia's east coast and in Mexico, the display of colors is absent when opal is viewed in transmitted light.
Precious Opal - top grade solid opal which displays one or more surface colors in a variety of patterns in reflected light. With the exception of some Queensland boulder opal, where an especially thick layer of precious opal has been deposited on ironstone, precious opal includes neither matrix nor rock. It is semi-transparent, at best, but is usually translucent or opaque. A cloudy milk-white or a slight blue or pearly grey body color is typical. A pronounced red, yellow, green, black, or blue body color is rare. In volume 2 of his book, PRECIOUS STONES, Max Bauer speaks of a "magnificent example" of very rare rose-red opal in the "Green Vaults" in Dresden, Germany.
Harlequin Opal - "True precious opal showing a regular mosaic-like pattern in rounded, angular, or roughly square patches of about equal size, presenting a spangled appearance--". This definition is found in A Field Guide To Australian Opals by Barrie O'Leary. He also states that "Harlequin opal is precious opal in which the colors shine as patches and spangles." The harlequin pattern occurs in other types of opal and should not be confused with solid true precious harlequin opal.
Black Opal - natural opal with a black, very dark grey, or dark brown body color. It absorbs most of the light striking its surface, thus, creating a dramatic contrast to the colors caused by the diffraction of reflected light. Another black opal is that formed by the natural deposition of transparent precious crystal opal onto black potch, allowing the body color to be clearly visible. O'Leary makes a point that the term," black opal", does not include matrix opal nor boulder opal.
Neither should the term apply to doublets or triplets created by the use of a veneer from black opal. Noble opal is the correct term to use in such cases if the quality warrants it.
Crystal Opal - that in which the play of color emanates from a transparent colorless body, frequently associated with black opal
Celestial Opal - precious opal.
Oriental Opal - Hungary was the source of nearly all of the best opal supplied to Europe in early times. It was usually sent from the mines near Czerwenitza to Constantinople and forwarded to various European cities.
This term came to be known as oriental opal. The Imperial Natural History Museum in Vienna has the largest known specimen from the source mentioned above. The stunning, nearly 600 gram wedge-shaped uncut opal may be the larger portion of a stone that yielded the remarkable hen-egg sized gem in the Imperial Treasury there.
Sedimentary Opal - opal deposited in the voids and crevices of sandstone formations. Until opal was discovered in Queensland, Australia in 1872, all opal was recovered from rocks of volcanic origin.
Opaline - a term formerly used by jewelers to denote Australian opal when it came on the market to distinguish it from the Hungarian opal.
Noble Opal - all brilliantly colored opal, other than solid precious opal. According to O'Leary, this includes top quality doublets and triplets.
Boulder Opal - opal found in ironstone concretions within sandstone formations. This opal often is so thin that the finished gem must include the natural matrix backing. Yowah nuts and pipe opal are types that yield solid opal. Small hollow ironstone concretions that resemble nuts were first found near Yowah, hence, the name. Sometimes these contain a solid core of precious opal. Long finger-like concretions, called pipes, yield precious opal of consistent quality. These can be cut into matching high-domed cabochons that are especially valuable.
Potch Opal- transparent material where the voids between the regular array of the microspheres have been filled with silica cement that prevents the diffraction of light and the play of color. Good grades of potch, both black and white, are used as backs in the manufacture of doublets and triplets. When partial cementation distorts and reduces the size of the voids and the power of diffraction, various types of opal can be the result.
Hydrophane - a porous potch exhibiting a weak play of color only when wet. Its porosity causes it to adhere to a moist surface.
Hyalite - a colorless transparent glassy potch.
Fire Opal - transparent amber to orange to red potch. It is often faceted and is very sensitive to heat and stress. Mexico is an important source. Glass imitations are marketed.
Cherry Opal -a rich red fire opal.
Girasol - a term, at times, applied to other gemstones, but here, it denotes a semi-transparent opal with a billowly blue sheen. The effect resembles moonstone and can also exhibit a red play of color.
Sun Opal - very transparent, bright yellow fire opal.
Mexican Water Opal - a colorless or pale brown opal exhibiting a single colored schiller.
Common Opal - mostly opaque without any play of color.
Agate Opal - interspersed layers of agate and common opal.
Milk Opal - white translucent opal with a pearly luster.
Porcelain Opal - opaque whitish opal.
Moss Opal - porcelain opal including dendrites.
Cacholong - white or yellowish, translucent to opaque, common opal with a mother of pearl luster. It is often dull. Because it is very porous, it adheres to a moist surface.
Honey opal - translucent yellow opal.
Prase Opal - apple green, semi-translucent to opaque, common opal. Once known as chrysopaI because of its resemblance to chrysoprase. Nickel gives it its color.
Wax Opal - also called resin opal; a waxy, lustered, opaque yellow- brown opal.
Menilite - liver opal; grayish brown, concretionary opal. It yields a brilliant luster when polished. An excellent picture of it is published in Walter Schumann's Gemstones Of The World, page 152.
Catseye Opal - material where the play of color forms an "eye" or a band. Tabasheer refers to opaline silica deposited in the joints of bamboo.
Opal Pseudomorphs - the deposition of opal in casts (molds) of fossil bone, teeth, shell, belemites (ancient relatives of the cuttlefish), crinoids (sea lillies), wood, fir cones, and even skeletons of large prehistoric animals. Many of these fossilized forms contain exceptional quality of noble opal. An especially interesting example is the opal "pineapple" found at White Cliffs in Australia. A pre-existing crystal of the mineral glauberite dissolved, resulting in a cast filled with opal.
From the proceeding definitions, one can begin to comprehend the vastness and the complexity involved in the study of opal. The sources are numerous, and the judgment of its quality and value is very subjective. An excellent source of concise information is Joel Arem's Color Encyclopedia Of Gemstones. Lack of space precludes a discussion of its synthesis (many are on the market) and its imitations. Much of the gemological information has already been well covered, so only the following data need be given.
Another opal reference book recommended is the book "Opals" by Fred Ward, G.G.
Variable. Strong white, medium blue, dull white, bright blue, pale yellow, brownish, bright green (indicates U minerals), brownish. Fire opal often greenish brown. Black opal usually inert.