COMMON OPAL . . . UNCOMMON BEAUTY

When it comes to the value and popularity of opal, the common variety has always taken a backseat to the precious variety. But while common opal lacks the glittering, multicolored opalescence of its precious counterpart, it is nevertheless a beautiful gemstone with its own unique qualities.

OPAL: PRECIOUS AND COMMON

Both the common and precious varieties of opal consist of hydrous silicon dioxide. Despite having a composition similar to that of quartz (silicon dioxide), opal’s properties are much dierent. Because of its attached molecules of water, opal is substantially soer, more brittle, and less dense than quartz. Most importantly, unlike quartz, common opal does not form crystals.

With its amorphous structure and variable chemical composition, all forms of opal are classied as mineraloids—natural, mineral-like materials that do not meet all the qualications of a mineral. But because opal was historically classied as a mineral, the International Mineralogical Association recognizes tradition and continues to list it as a valid mineral species.

OPALESCENCE . . . AND THE LACK OF IT

Opal is a solidi ed, colloidal silica gel with an amorphous structure consisting of randomly arranged, interspersed layers of hydrated silica molecules and microscopic spherules of anhydrous silica. e opalescence of precious opal is created when layers of microscopic silica spherules di ract white light into its component colors. When re ected, these diffracted light waves become reinforced to produce the phenomenon of optical interference and the iridescent colors seen in precious opal.

Common opal either lacks silica spherules or consists of spherules that are too large or too small to create opalescence. Although lacking opalescence, common opal has varying degrees of translucency and scatters light internally to produce a so, warm glow similar to that of moonstone. Precious opal is by far the rarest variety of opal, followed by gem-quality common opal. Non-gem-quality common opal is fairly abundant and occurs as fracture and seam fillings, grain cement in sedimentary rocks and even as opalized wood.

While some common opal is white or colorless, most exhibits a color range caused by traces of accessory elements, usually iron. e colors of common opal include orange-red re opal, white milky opal, green prase opal, Peruvian pink and blue opal, yellow honey opal, brown wax opal and yellowish-green hyalite.

FIRE OPAL

e best-known and most valuable type of common opal is re opal; its rich, orange-red color is caused by microscopic inclusions of hematite. In describing opal colors, “re” can refer to the opalescence of precious opal or to the orange-red color of common re opal.

In terms of both quality and quantity, the best re opal comes from central Mexico. Because of its unusual degree of transparency, Mexican re opal is usually faceted, while all other types of common opal are cut as cabochons. Mexican re opal occasionally exhibits subtle ashes of greenish-yellow opalescence because of partially formed, silica-spherule layers that di ract and re ect only part of the incident light.

HYALITE

e only truly transparent common opal is hyalite, in which subtle hints of greenish-yellow are caused by traces of iron. Hyalite also exhibits the rare phenomenon of “daylight fluorescence” which occurs

Although lacking opalescence, common opal has varying degrees of translucency and scatters light internally to produce a soft, warm glow similar to that of moonstone.”

when sunlight energizes electrons within the opal. ese electrons then release excess energy as visible, greenish-yellow light. e combination of daylight fluorescence and hyalite’s base color produces a particularly pure and lively greenish-yellow hue. Hyalite’s daylight fluorescence and its strong, ultraviolet fluorescence are because of traces of the radioactive element uranium. While most opal solidifies directly from silica gel, hyalite forms when silica-laden gases emitted by rhyolitic magma condense into silica gel that subsequently solidifies into botryoidal masses. Although hyalite rarely occurs in sizes large enough to facet, botryoidal free forms are often wire-wrapped and worn as pendants.

OPAL FROM PERU: PINK AND BLUE

Peruvian pink opal, a rare example of organic coloration in gemstones, consists of a mixture of hydrated silica, chalcedony, and palygorskite, the latter a complex hydrous magnesium aluminum silicate. Palygorskite’s brous crystals have a strong anity for organic molecules.

The pigments in Peruvian pink opal are reddish-colored quinones, hydrocarbon compounds that originated as ancient plant materials in a now-buried lake bottom within a basaltic environment. With an abundant supply of magnesium from the

basalt, palygorskite formed, attached quinone molecules and became part of the silica gel that solidi ed into Peruvian pink opal. Peruvian blue opal, characterized by its blue-to-blue-green color and so translucency, comes from an old copper-mining district in southern Peru. Its unusual color is produced by microscopic inclusions of the copper-bearing mineral chrysocolla.

OTHER COMMON OPALS

Milky opal, not to be confused with white precious opal, is the most abundant type of gem-quality common opal. It has an o-white color, a moonstone-like translucency, and a warm glow caused by the internal scattering of light. e presence of iron o en produces so , bluish or orange undertones. Milky opal is fashioned into beads and cabochons. Prase opal’s distinctive, chrysoprase-like green color is caused by a nickel chromo-phore. The dark, spiderweb-like patterns of dendritic opal are created when black manganese oxides ll tiny fractures in the crazed (fractured) opal surface. Multicolored, banded opal, which has only limited translucency, forms from variations in the chemical composition of silica gel during the solidi cation process. So while precious opal may always rule the roost in the opal hierarchy, common opal is nevertheless a ne and distinctive gemstone—even without opalescence.

The best-known and most valuable type of common opal is fire opal; its rich, orange-red color is caused by microscopic inclusions of hematite.”

Steve Voynick Mexican fire opal occasionally exhibits subtle flashes of greenish-yellow opalescence due to partially formed silica-spherule layers.

Steve Voynick The only truly transparent form of common opal is hyalite; its subtle hints of greenishyellow are caused by traces of iron.

Wikimedia Commons Peruvian pink opal is a rare example of organic coloration in gemstones.

Wikimedia Commons The color of Peruvian blue opal is due to tiny inclusions of the copperbearing mineral chrysocolla.

Steve Voynick White milky opal, the most abundant type of common opal, has bluish or orange undertones and an attractive, moonstone-like translucency.