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Suzanne Paradis, Geological Survey of Canada, Sidney, BC.
George Simandl, British Columbia Geological Survey, Victoria, BC
Opal is hydrated amorphous silica (3% to 20% water) containing aggregates of ordered or disordered microcrystallites of a cristobalite. Electron microscopic studies show that opal is made up of closely packed silica spheres (SiO2·nH2O) and interstitial silica, water or CO2 gas-vapour air. Opal is characterized by a conchoidal fracture and it occurs in wide range of background colours. It is transparent to nearly opaque, and may be distinguished from chalcedony and cryptocrystalline varieties of quartz by its higher water content and lower hardness. Opal is brittle, sensitive to heat, and easily scratched. Furthermore, opal from some localities, is "unstable" and may crack or self-destruct through the loss of water. Despite these faults, opal's beauty is supreme and, for the past thousand years, it is recognized as a "highly prized" gemstone.
Precious Opal in Basalt. Vernon, British Columbia. University of Waterloo Earth Sciences Museum Collection. Found by Glen, Darryl, and Carolyn Grywacheski
The word "opal" evolved from the Roman "opalus" or the Greek "opallios" meaning "to see a change of colour". The Greek version is a modification of the ancient Indian Sanskrit name for opal "upala", which meant "precious stone". As suggested by the derivation of its name, opal has centuries of history as a treasured gemstone. The early Greeks thought that opals gave their owners the powers of foresight and prophecy, and the Romans adored it as a token of hope and purity. Eastern people regarded it as sacred, and Arabs believed it fell from heaven. In the nineteenth century, superstitions grew about the bad luck or fate that could befall one for wearing opal if it were not the wearer's birthstone. Today, these superstitions have diminished, but some people still believe it is bad luck to wear opal.
Opal may be deposited from silica-bearing hydrothermal solutions in hotspring or geyser environments, from meteoric waters or as accumulations of tests of silica-secreting organisms called diatoms in marine or lacustrine environments.
The terminology currently used to describe opal for the lapidary applications is quite complex. The three most commonly used terms are "precious", "common", and "fire" opal. Precious opal is defined as opal with a bright, internal play of colours that may be red, orange, green or blue. This play of colour is caused by diffraction of white light by regular packing of silica microspheres within the mineral structure. The diameter and spacing of the microspheres controls the colour range of an opal (Darragh et al., 1966). Precious opal may be subdivided further by colour modifiers, white, black, pink, and blue, which describe the body colour of the opal. Australia is famous for its white and black precious opal. The term "common opal" groups all opals without a play of colours (including the fire opal). The lack of play of colours may be due to less ordered packing of the silica microspheres. Fire opal is defined as a solid opal with transparent orange to red-orange base colour; it belongs to the precious opal variety if it shows a play of colour or to the common one if it lacks play of colour. Some of the best fire opal comes from Mexico.
Worldwide, deposits that contain precious opal can be divided into two major categories based on host lithologies: sediment-hosted and volcanic-hosted. Australian deposits of the Coober Pedy, Andamooka, and Mintabie areas are excellent examples of sediment-hosted fields. The deposits in these areas are believed to have formed by descending silica-bearing meteoric waters. The source of silica is linked to the intensity and depth of weathering in the sediments overlying the opal deposits. The silica in meteoric waters was concentrated by evaporation resulting in the formation of colloidal silica gel and ultimately opal. Deposits such as those of the QuerŽtaro region of Mexico, Gracios ˆ Dios area in Honduras and Spencer in Idaho, USA are excellent examples of volcanic-hosted opal.
Most of the world's gem opals are from the Australian fields of Andamooka, Coober Pedy, and Lightning Ridge. The remaining production comes from the volcanic-hosted deposits, such as those of Mexico and Honduras. The volcanic-hosted opal deposits are believed to be genetically associated with hydrothermal activity.
In British Columbia, common opal occurrences are relatively widespread within tertiary volcanic rocks (Leaming, 1973), but precious or gem-quality opals are rare. Some of the most interesting opal occurrences are Klinker, which contains precious opal (Simandl et al., 1996), Queen containing common opal (Church and Hora, 1996), and Eagle Creek in south central British Columbia.
Precious opals recently found at Klinker, approximately 25 kilometres N-W of Vernon, British Columbia.
Recently, beautiful precious opals (Figs. 1 and 2) were found at Klinker located at approximately 25 kilometres northwest of Vernon, British Columbia. Access to the prospect is via a forest service road that connects the main pits at 1475 metres elevation to Highway 97 in the Okanagan Valley to the east. The Klinker deposit (Fig. 3) is the first precious opal occurrence considered for commercial development in Canada. The opal discoveries are well exposed in a clear-cut logging area and several shallow pits. Opal is hosted by volcanic lahars (i.e., a debris flow composed of clast- and matrix-supported volcanic materials mixed with water), scoriaceous beds or blocky lava flows, and ash and lapilli tuffs (i.e., pyroclastic rocks whose average pyroclast size is < 64 mm) of Eocene or Miocene age (Simandl et al., 1996). The precious opal coexists with common opal and agate, which occur as open-space fillings, mainly within fractures, voids, and vesicles in the volcanic rocks. Precious opal occurs in altered or weathered ash to lapilli tuffs and adjacent matrix-supported lahars that dip shallowly (0 to 20°) and closely follow the underlying topography. Consequently, the most important control of opal distribution at Klinker is stratigraphy, structure, and rock porosity.
Precious opals obtained from the Klinker deposit have excellent brightness and multicolour "flash" and "broad flash" patterns. The base colour may be water-clear, orange, honey, red-brown, orange or white. Clarity of the stones varies from transparent through translucent to opaque. Non-precious, facet-grade opals are typically orange and honey colours, similar to the Mexican fire opal (clear reddish-orange opal; some show a play of colours). Common opals occur as transparent, translucent and opaque types in white, honey, brown, amber, orange, and grey.
The Klinker deposit is currently under evaluation for commercial mining, however it is also open to fee digging during the season (mid-June to mid-October). For more information about organized visits or fee digging at Klinker, contact Bob and Alana Yorke-Hardy, Okanagan Opal Inc., P.O. Box 298, Vernon, British Columbia, V1T 6M2 (Tel: (250) 542-5173 or 542-1103, Fax: (250) 542-7115).
The Queen and Eagle Creek occurrences in south central British Columbia are known to contain large size specimens of transparent, translucent or opaque varieties of common opal. The potential of these occurrences in terms of precious opal is not fully determined yet. The Queen claims are located on Nipple Mountain, approximately 35 km southeast of Kelowna. The property is owned by Donald Sundberg at Kelowna (Tel: 250-765-6137).
The Eagle Creek occurrence is commonly referred to as "Eagle Creek Agate and Opal Beds". It is located in central British Columbia, approximately 6.5 kilometres from Burns Lake. This site is underlain by nearly flat-lying volcanic rocks that probably belong to the Tertiary Endako Group. Walking time from the picnic area to the opal/agate bearing outcrops is about 30 minutes, but it involves some steep climbs. Mineral collecting is permitted in this area which has been withheld from staking by the town of Burns Lake, having turned it into a park. Agate nodules and common opal are abundant. Rare fire opal but no precious opal was found during our brief visit. Further information on this site may be obtained from Burns Lake Chamber of Commerce (Tel: 250-692-3773, Fax: 250-692-3493)
In British Columbia, the wave of systematic prospecting for precious opal was sparked only after the discovery of the Klinker deposit, in 1991. There is no doubt in our minds that new common and precious opal occurrences will be found during the ongoing wave of exploration.
Glossary of terms:
Agate: A translucent cryptocrystaline variety of silica frequently mixed or alternating with opal and characterized by colors arranged in alternating stripes or bands, in irregular clouds or in moss-like forms.
Cristobalite: A high temperature polymorph of quartz. The (-cristobalite is stable at lower temperatures than the (-cristobalite.
Breccia: A coarse-grained clastic rock, composed of angular broken rock fragments cemented by a fine-grained matrix.
Cryptocrystalline: Texture of a rock consisting of crystals that are too small to be recognized and distinguished even under optical microscope.
Conchoidal: Smoothly curved fracture surface.
Hydrothermal: Pertaining to precipitation of minerals from aqueous fluids of magmatic or meteoric origin.
Lahar: Muddy matrix-supported rock consisting mainly of volcanic rock fragments and soils derived from the volcano. These form as mudflows in drainage peripheral to volcanoes.
Lapilli tuff: An indurated deposit where pyroclastic material (2 to 64 millimetres) is cemented by finer-grained matrix.
Meteoric: Pertaining to shallow ground water of recent atmospheric origin.
Microcrystallite: Material where the mineral composition of a rock is discernible only under optical microscope. Scoriaceous: A texture of coarsely vesicular pyroclastic rock.
Tertiary: A period of time between 65 and 2 million years ago.
Tuff: A general geological term referring to all consolidated mostly fine-grained pyroclastic rocks.
References
Darragh, P.J., Gaskin, A.J., Terrell, B.C., and Sanders, J.V. 1966: Origin of precious opal. Nature, January 1, 1966, Number 5018, p. 13-16.
Church, B.N. and Hora, Z.D. 1996: New Splitstone and Opal Occurrences, Nipple Mountain, Beaverdell area; in Geological Fieldwork 1996, British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, Paper 1997-1, p. 329-332.
Leaming, S. 1973: Rock and Mineral collecting in British Columbia; Geological Survey of Canada, Paper 72-53, 138 pages.
Simandl, G.J., Hancock, K.D., Callaghan, B., and Paradis, S. 1996: Klinker precious opal deposit, south central British Columbia, Canada - Field observations and potential deposit-scale controls, in Geological Fieldwork 1996, British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, Paper 1997-1, p. 321-327.