Opals form on all continents of the globe and have been sought after worldwide since ancient times.
Whereas, the Australian opal fields are founded upon sedimentary geology, opals found outside of Australia are predominantly volcanic in origin.
Sea Plankton share the same chemical composition as common opal. Consequently, opal is being deposited on the deep ocean floor where dead Plankton sink and collect everyday.
Since the 12th century AD, right up until the early 19th century, world markets were supplied with precious opal found at the Dubnik mines (now in eastern Slovakia, once part of the Austro-Hungarian Empire).
The Aztecs had first mined opal in Central America by the 16th century. Some fine pieces were sent back to Europe by the Spanish Conquistadors who also worked the deposits. The occurence of precious opal throughout Mexico and Honduras is found in a belt of volcanic rock stretching all the way from Canada. Coincidentally, most of the opal deposits on both the North and South American continents are situated along the Continental Divide. Including the first major modern discoveries which were made some time after 1851 in Mexico's Querétaro state. However, the discovery of White Cliffs opal field in 1889 saw Australia become the world's largest opal producer by the turn of the 20th century. Mexico and Brazil have remained important second tier commercial producers ever since. Honduras, USA, Canada and Indonesia have been niche producers for more than a century.
At the dawn of the new millennium; Ethiopia has emerged as a first tier producer with huge prospective reserves of opal. Potential opal deposits have even been identified by NASA on the planet Mars and Stanford researchers propose to use opal to decontaminate nuclear sites.
The Boi Morto Opal Mine near Pedro II in the northeastern state of Piaui is the largest working opal mine in Brazil. Largely developed by Australians (Christianos Family & Neville Carter) from the early 1970's to the mid 1980's, the main diggings at Boi Morto are over 120 foot deep.The garimpeiros (miners) of Pedro II, Brazil, hardly operate heavy machinery on these opal fields which are largely excavated manually. Explosives are used to break up the hard rock and pumps are used to remove the rain and seepage water.
Opal Cat's Eyes; yellow-honey coloured stones were discovered in the state of Bahia Brazil in 1991. They do not exhibit the 'play of colour' phenomenon however displaying the phenomenon known as chatoyancy, the gemmological term from the French for “the cat's eye effect”. The cause of this are numerous parallel aligned fine needle-like inclusions within the stone. When a light source is directed on these inclusions it creates a bright strip that appears, running perpendicular to the inclusions. This strip will then glide across the surface of the stone when it is turned or tilted over. The most important factor to consider when evaluating an 'Opal Cat's Eye' is the strength and sharpness of the eye. The eye of the stone must have a fine line running through it that is distinct and easily recognized. In addition to this, the body color is an important factor that has a significant effect on value. Opal cat's eyes are almost exclusively cut into high-domed cabochons so as to maximize the striking chatoyant effect most brilliantly.
There are numerous occurrences of opals in Mexico, ranging from just south of the United States border (where it occurs as a translucent sky blue variety in the state of Sonora and a non-precious cherry opal in Chihuahua) to just north of the Guatemala border where crystal and black opal occurs in the states of Oxaca and Chiapas. Black opal also occurs near Huitzuco in the state of Guerrero.
Located in the San Juan del Rio region of Querétaro state the Santa Maria Iris mine has been worked steadily since 1865 and is still productive, as is the Magdalena mine in Jalisco state which has been in operation since 1962.
Most of southwestern Honduras, particularly the Lempira region where most of the opal deposits are located is covered by 4000 feet of volcanic flows, dating to Pliocene times. The opal bearing material was deposited over ever older formations dating from the late Paleozoic to late Cretaceous. The volcanic belt extends southward from the Continental Divide but also to a limited extent to the north. Although this formation extends into Guatemala, El Salvador and Nicaragua newer volcanic rock covers it. Whereas Honduras, which has no active volcanoes, managed to retain the original opal bearing strata near surface.
In the vicinity of Erandique are the most extensive opal deposits in Honduras, mines include; Valle de la Cruz, Cuesta de la Mina, Yolomon, Cuevas del Tablon, La Esperanzita, Jupual, Azacualpa Grande, El Barranco and Peladera. Here the opal is found in seams or veins that penetrate the dark Basaltic Andesite in near vertical fashion in veins upto 2 feet long. Sometimes the opal fills irregular gas holes and horizontal cracks as well. On rare occasions the irregularly shaped masses filling the gas holes can yield thick gems of suberp quality. Some Erandique opal has small shafts of vertical fire, comparable to Mexican Lloviznando Opal, although rarely encountered and not usually as fine as the Mexican variety. Other varieties include Milky Opal, Water Opal, Contra Luz and Matrix Opal often referred by the misnomer "Black Honduran Opal"(pictured right). Sometimes opals with a dark brown or amber base are found. Precious opal is associated with a substantial amount of chalky hydrophane opal often found sandwiched between the matrix and the precious opal.
The opal deposits do not appear in Honduran public record until 1843. However, precious opal was known to the pre-colombian indians prior to the arrival of the Spanish Conquistadors. The Maya and Leuca tribes had empires whose influence extended into the opal bearing region of Homduras.
According to the United States Geological Survey there are many sources where opal is mined in the United States.
- • Arizona – produces precious blue opal with strong play-of-color.
- • Idaho – produces precious white and pink and pink, yellow blue and common opal. Opal occurs in thin layers and seams and only 10% is thick enough for solid gems, the rest is manufactured as doublets and triplets.
- • Louisiana – opal matrix or cement within a sandstone/quartizite mix. Often cut into large spheres and displaying blue and purple play-of-color.
- • Nevada – precious opal has been mined in the Virgin Valley for over a century. This opal occurs primarily as a replacement of wood or conifer cones. The material is very brittle and crazes easily. Usually found as specimens displayed in liquid.
- • Oregon – fine quality gem material including hyalite, rainbow, contra luz, hydrophane, crystal, fire, blue, and dendritic. Opal is found in rhyolite geodes (thundereggs). Of these geodes only about 10% of the total geodes contain opal and of those only 1% contain gem-quality opal with play-of-color. The other geodes contain agate, quartz crystals, or common opal.
Klinker, Canada's first commercial grade precious opal mine, was discovered near Vernon, British Columbia in October 1991. The opal from this site was given the name "Okanagan Opal" by the prospectors who found this 'Mountain Opal' deposit. Klinker attracted significant local and international attention through the publication of several articles written by opal expert Paul Downing, through a government paper published in 1997 and through numerous other press articles. The Klinker Opal deposit has been undergoing exploration and development since the summer of 1993. The size and full economic potential of the deposit is as yet undetermined however the development which has taken place indicates that this is a significant albeit remote discovery. The precious opal occurrences are controlled by fracturing, stratigraphy and porosity within shallow dipping, mid Miocene, Ash/Lapilli Tuff volcanic rocks.
|Peruvian Blue Opal and Pink Opal are monochromatic, translucent to opaque gemstones which do not exhibit play-of-colour.|
At the dawn of the 21st Century there is a new contender as a major producer of opals in global terms. Although Ethiopia has produced Opal for well over a decade, Wollo (Welo) discovered in 1993 is the most stable opal find to date. The Opals from Gondar and Wollo (Welo) provinces are on the Abyssinian highland plateau of the Amhara region which lies 7500 to 10000 feet above sea level. Ethiopian opals are region specific in their character traits just like Australian opals.
The colour is brilliant and may rival any top grade opal in the world. Most have a brightness level of at least 4 to 5 on the brightness scale with multi-coloured and multiple patterns in stones. Welo opal is not generally classified as 'contra luz' opal although many are examples. The colour play is face up and mostly as bright in artificial light as it is in direct sunlight. However, Welo opal is hydrophane which requires patience and some special cutting techniques; Hydrophane opal when soaked in water causes the base colour to clear up sometimes highlighting the play-of-colour, sometimes making it vanish. If soaked in water, it will take one to two weeks to completely dry out and return to its original beautiful state, without use of artificial means.
The Ethiopian Government does not own any mines. Yet the land is owned by the government, it is administrated by the regional authorities in Amhara who do not allow mining other than by registered and licensed farmer co-operatives.
The world's first commercial opal production began at the turn of the 12th century originating in the heart of the northern part of the Slanské vrchy Mountains in Dubnik Slovakia. Dubnik remained the seat of world opal production until the turn of the 20th century when Australia became the world's largest opal producer.
Volcanic Opal in Australia
Opal on Mars in Outer Space
In October 2008 NASA made a press release regarding the Mars Reconnaissance Orbiter having observed a new category of minerals spread across large regions of Mars. This discovery suggests that liquid water remained on the planet's surface a billion years later than scientists had initially believed, and it played an important role in shaping the red planet's surface and possibly hosting life.
Researchers examining data from the orbiter's Compact Reconnaissance Imaging Spectrometer for Mars have found evidence of hydrated silica, commonly known as opal. The hydrated, or water-bearing, mineral deposits are indicative of where and when water was present on ancient Mars.
Research that sheds light on how Australian opals formed indicates the red dirt of the outback might be similar to the red surface of Mars. Acidic oxidative weathering has also been seen on the surface of Mars, which shares a number of other features with the Great Artesian Basin. "Similar rocks, similar flooding then drying out history, similar mineralogy, the same red colour," University of Sydney Professor Patrice Rey said. "This story shows that space exploration helps us to better understand our own planet."
All in all, the red center of Australia could be among the best matches on Earth for the surface of the red planet.
"It costs billions of dollars to send rovers and orbiters to Mars," Rey said. "Therefore, looking right here on Earth for ancient and modern analogues to Mars' environment is key to carry on research in greater detail and explore the role biology has in weathering processes."
Opaline Remedy for Radiation
Besides being gemstones, opals could be used to decontaminate nuclear sites; Cleaning up nuclear sites where soil and groundwater are contaminated with substantial amounts of uranium (a highly mobile and radioactive element) is a problem that has bedeviled remediation efforts for decades. There has been no simple, reliable or cost effective method.
Since 2011 a team of researchers led by Stanford University geochemist Kate Maher has proposed to imitate nature by using amorphous silica, also known as the precious gemstone opal, to sequester the uranium. Once ensconced inside opal, the uranium molecules would be rendered immobile and chemically inert.
We have looked at opaline silica in deposits across the western United States and almost universally we find very high uranium concentrations,...From dating these deposits, we have found that they have been stable, closed systems for hundreds of thousands and in some cases millions of years - Prof. Kate Maher
Whether in soils, hydrothermal deposits, hot spring or cold spring deposits, when enfolded in an opaline embrace, uranium seems about as active as a bug trapped in amber. According to computer modeling studies that the researchers have done using their data from natural opal deposits, opaline silica may offer a faster, cheaper, more enduring way to sequester uranium than other current or proposed methods. The sequestering process would involve pumping a solution rich in dissolved silica into the subsurface through injection wells, effectively flooding the contaminated areas with it. As the solution moved through the soil or rock, chemically interacting with its surroundings, amorphous silica would precipitate out and latch on to dissolved uranium.
Approaches for in situ remediation generally involve reducing the electrical charge of the uranium atoms, and thus their chemical reactivity, by means of various biological or chemical agents. Certain microbes have had some success in reducing uranium to a stable state. However, silicate minerals are the most abundant class of rock-forming minerals in the crust of the Earth, composing about 90 percent of the crust, and in many geologic environments most of the waters are close to saturation with silica. This makes the researchers confident that opaline silica will be stable over long time scales. Silica is also relatively inexpensive, making it an affordable method for storing in situ in the subsurface. So far the researchers’ work has been focused on sampling and analyzing naturally occurring deposits of opal and using that data to model the reactivity and transport of uranium under different scenarios. They are particularly interested in how iron oxides, which are commonly present in soil and sediment, might affect the incorporation of uranium into opal.
References & Image Credits:
THE INTERNATIONAL OPAL JOURNAL, Vol.1. No.1, 1977. (Article: Opal in the United States and Canada, By Catherine J. Gaher)
EXTRA LAPIS, No. 10. OPAL THE PHENOMENAL GEMSTONE, 2007. (Map by Gunter Neuemeier)
OPAL REPORT FROM HONDURAS, Tony Dadoub, 1985.
THE WORLD OF OPALS, Allan.W. Eckert, 1997.
addisfortune.com (Welo:The circle area on the picture is where the semiprecious gems are found)