OPAL: QUEEN OF THE GEMS

Precious opal is one of those gems that I really never tried to collect.  It fractures rather easily, often cracks with dehydration, and is expensive.  And, most collectors (unlike me) are lapidaries and make wonderful items from the stone.  However, at a recent estate auction I picked up a particularly gorgeous specimen in matrix.  I don’t have the slightest idea about provenance but suspect Australia.  At any rate, I enjoy observing the specimen.

Precious opal, provenance unknown (perhaps Australia).  Width 4.3 cm.

Opal is a mineraloid, a mineral-like substance that does not have a definite chemical formula nor a definite crystalline structure (although some opal has a loose-ordered arrangement of the silica spheres).  Perhaps the best known common mineraloid is obsidian, an amorphous volcanic glass.  Opal is hydrous silica, that is, silicon dioxide (like quartz family minerals) but with an indefinite amount of water (up to perhaps 18%-20%) in its atomic structure.  Eckel and others (1997) noted that new studies using an X-ray Diffractometer (XRD) show that opal commonly contains significant amounts of the high temperature polymorphs (same chemistry, SiO₂) of quartz known as cristobalite and tridymite. The chemical formula for opal is written as SiO₂-nH₂O where n represents the variable amount of water.  In the real world, the more water opal contains the more likely a chance for desiccation and cracking (termed crazing).  Opal comes in a variety of colors, has a hardness of ~5.5-6.5, a waxy to dull to greasy luster, and a white streak.  It also “feels light” as its specific gravity of ~2.15 is less than the specific gravity of quartz at ~2.65.  I often find it difficult to identify common opal from other siliceous minerals such as chalcedony and agate; however, the key points seem to be the waxy/dull/greasy luster, and the low specific gravity.  There is a plethora of names (well over 100 that I have located) assigned to opals with various colors and from different collecting localities.  However, most collectors would differentiate opal into three broad groups: precious opal, common opal (including hyalite), and fire opal.

Of course, precious opal is the most valuable of the group and specimens are characterized by a “play of colors”, that is a flash of colors (almost every color “in the rainbow”) when moved and rotated.  This play of colors seems due to the refraction, reflection, and diffraction of light as it passes through the internal structure of somewhat ordered silica spheres (Klein and Hurlbut, 1985).  The best known collecting localities for precious opal are Coober Pedy and Lightening Ridge, Australia.  The latter locality produces black opal that exhibits a play of colors with red, green, blue, violet, magenta or yellow against a dark background.  In addition these Australian opals are valued for their stability (low water content).

In the U.S., the Virgin Valley of Humboldt County, Nevada, produces a fantastic array of precious black opal, much of it being opalized conifer wood, a pseudomorph after the original wood.

I have found references to the following varieties (and several more) of precious opal: white opal (most common, white or cream stone color), black opal (dark stone with a strong play of colors), crystal opal (transparent to translucent stone), boulder opal (opal in veins), harlequin opal (play of colors in rectangular shapes), pinfire opal (play of colors in small points), and cat’s eye opal (play of colors in a “cat’s eye”).  In order to best display the play of colors, most precious opals are cut into cabochons, rather than shown faceted.

The famous opals of Nevada were officially recognized in 1987 when the State passed the following:  NRS 235.100 State precious gemstone. The precious gemstone known as the Virgin Valley black fire opal is hereby designated as the official state precious gemstone of the State of Nevada.  Australia went even a step further when the Commonwealth, in 1993, declared opal as the national gemstone.

Besides Nevada, other U.S. states producing precious opal on a commercial scale include: Arizona (two mines producing blue precious opal), Idaho (second in production to Nevada), known for pink precious opal, Louisiana (sandstone with precious opal cement), and Oregon (from geodes or “thundereggs”) (U. S. Geological Survey, 2002).  Many other states produce precious opal on a small scale collector or specimen basis.

Faceted fire opals from Mexico.  Photo courtesy of International Colored Gemstone Association.

Fire opals usually do not display a play of colors (some brown-tones may be an exception), are translucent to transparent and most often come in a variety of “earth tones”—yellow, oranges, and reds, most likely due to the presence of minute amounts of iron oxide.  The most famous collecting localities for these gems is in the State of Queretaro, Mexico, although Australia is now producing significant quantities and Brazil has opened a mine producing orange and yellow stones. In the U. S. Oregon is producing orange fire opals.  Not all fire opals need cutting into cabochons as many display facets quite nicely.

Very gemmy common opal, provenance unknown (perhaps Peru).  Width 7 cm.

Reverse of above figure showing polish and dendrites.  White light reflecting as spot.

Common opal does not show a play of colors but often displays opalescence, a sort of sheen on an otherwise chalky to transparent to translucent siliceous material.  Opal is a mineral that is hard to describe; however, once you observe the gem it becomes recognizable (mostly!).  Common opal usually is not considered a gemstone and according to some “authorities” is of little or no value (Oldershaw, 2004).  However, beauty is in the eye of the beholder and I sort of enjoy common opal and have seen some really nice polished pieces.  Milk opal (commonly called potch), quarried from Australia, is sometime gemmy since it usually has a bluish opalescence.  Wood opal, if not precious opal, can still display nice colors.  Menilite (liver opal) is a grey to brown common opal evidently occurring only in scattered European localities (MinDat, 2010).  Hyalite, sometimes referred to as water opal or Muller’s glass, is a mostly colorless variety of opal usually found in globular concretions (MinDat, 2010).  I have never observed hydrophane, an opaque variety that is highly porous and which turns more translucent or more transparent when immersed in water.  Resin opal is a darker colored common opal with a resinous luster.  Geyserite is an opal deposited around hot springs such as those found in Yellowstone National Park and is often confused with travertine (CaCO₂) springs.  In fact, most Yellowstone travertine is found at Mammoth Hot Springs while the remainder of the springs produces geyserite.  Diatomite or diatomaceous earth, is a sedimentary rock composed of the fossilized remains of diatoms, a type of algae with an opalized skeleton.  Interestingly, this nondescript opal has by far the most value since the uses in industry are enormously varied—from filters to insecticides to cat litter.  In this part of the country the best known beds of diatomaceous earth are from the Tertiary Ogallala Formation of the High Plains.  These diatoms evidently lived in warm fresh water lakes impounded in the vast fluvial system of the late Tertiary.  Wallace County, Kansas, bordering Cheyenne County, Colorado, produced diatomaceous earth for many years from an open pit mine in the Ogallala.  The locality is well known to vertebrate paleontologists as the lake sediments also have produced a nice vertebrate fauna, evidently animals “washed” into the lake by area streams.

Opal can form in a variety of environments.  The famed Australian deposits have formed in Cretaceous sedimentary rocks as weathered silica collected in fissures, holes and other hollow spaces (a post-Cretaceous secondary formation of the opal).  The original source of the silica was feldspar-rich sedimentary rocks with normal weathering producing a silica gel.

One may envision how opal might form by purchasing sodium silicate (Na₂SiO₃), or water glass, from a pharmacy, and then combining the substance with vinegar (a weak acid).  The silicate reacts with the hydrogen of the vinegar to form silicic acid which turns into a hard glassy substance as water evaporates.  If the evaporation is rapid, numerous cracks will appear.  Slow evaporation and the substance will be rather solid.  This is a situation similar to the formation of opal---slower is better!

Additional formational types of opal include: deposition of silica from hot water, the geyserites; leaching of silica from volcanic ashes; aqueous solutions percolating through organic matter, such as wood, with subsequent deposition, etc. Opal also occurs as a vein mineral in ore bodies or as amygdule fillings in volcanic rock, mostly rhyolite (Eckel and others, 1997).  Opal is rare in metamorphic rocks.  Most opal is very young (geologically speaking) since it cannot withstand the heat and pressure associated with burial and metamorphism—the water is lost.  I am guessing (I am not a chemist) that dewatered opal “becomes” a form of microcrystalline quartz such as chalcedony.  Somewhere in my mind is a stored factoid that no opal is older than the Triassic (came from a class somewhere in the past); however, I could not locate a valid reference. 

Eckel and others (1997) noted that common opal has been found at a variety of localities and different geological environments across Colorado, and I refer the reader to that wonderful publication.  Very few localities in the state have produced significant amounts of gem opal or fire opal.  Of interest to this article, however, is the common opal occurring in the upper Tertiary Ogallala Formation.  In general, the Ogallala was deposited in a series of streams, flood plains and lakes extending eastward from the front ranges of the Rocky Mountains, an area now known as the High Plains.  Eckel and others (1997) listed a locality about 20 miles north of Burlington in Weld County, Colorado, that produced “moss opal”.  I have collected from this area and the material is rather poor, at least in the seams that I observed.  However, areas across the state line in western Kansas areas have produced very nice specimens of moss opal (also known as moss agate).  Within the last few months I have run across outcrops of the Ogallala that produced really nice specimens of opalized nodules that almost have a gemmy clear variety of opal.

Moss opal or moss agate collected from Gove County, Kansas. The specimen slice does not exhibit much "opalization" and is more of a dendritic chalcedony.  Width ~4.3 cm.

Outcrop of Ogallala Formation, Wallace County, Kansas.  The more resistant beds contain a silicified sandstone/conglomerate as well as opalized nodules. 

Silicified beds in the Ogallala have been known since the early part of the 20^th century, mainly from Kansas, but also common in parts of Nebraska and Texas.  The best known of these silicified beds is the aptly named “Green Quartzite”, a quartz to opal cemented sandstone and/or conglomerate that forms the local ”caprock” in many western Kansas localities.  Of greater interest to this study are the concretions and beds described by Frye and Swineford (1946) as irregular masses (up to 8 inches in long diameter) of dense, cream-colored, waxy or resinous opal …containing  vugs lined or filled with the more common translucent opal and some chalcedony, and on the outside consists of dull white porous silica… The rock is brittle and breaks easily with pronounced conchoidal fracture into small splinters.  The current thought is that the source of silica was the vast beds of volcanic ash scattered throughout the Ogallala.  Essentially this opal is a weathering product---silica leaching downward from the overlying ash beds.

Nodule of almost pure opal collected from the Ogallala 
Formation south of  Wallace, Kansas.  The transparent or 

clear opal is almost gemmy.  Specimen is about 5 inches in 

length.

The opalized nodules south of Wallace, Kansas, are among the most beautiful of the opalized concretions that I have observed.  Some of the translucent opal is almost, or may be, gem quality.  Although small, the “moss” dendrites (manganese oxide) are also present. 

Nodule of opal collected from Ogallala Formation south of   
Wallace, Kansas.  Penny for scale.  

Oh, give me a home where the buffalo roam
Where the deer and the antelope play
Where seldom is heard a discouraging word
And the skies are not cloudy all day

Dr. Brewster Higley penned these words describing western Kansas in a poem that later became the official State Song---Home on the Range.  I distinctly remember that students could not “pass” fifth grade until we were able to recite the entire poem (luckily I did not need to sing the song). 

REFERENCES CITED

Eckel, E.B. and others, 1997, Minerals of Colorado:Fulcrum Publishing, Golden, Colorado. 

Frye, J. C. and Swineford, A., 1946, Silicified Rock in the Ogallala Formation:  Kansas Geological Survey, Bull. 64, Pt. 2.

Klein, C, and Hurlbut, C.  S., 1985, Manual of Mineralogy, 20th ed.: John Wiley and Sons.

MinDat, 2010, Mineral Database: http://www.mindat.org/index.php.

Oldershaw, C., 2004, Guide to Gems: Firefly Books Inc.  Buffalo, NY.

U. S. Geological Survey, 2002, An Overview of Production of Specific U.S. Gemstones: U.S. Bureau of Mines Special Publication 14-95.