OPAL
Wayne Farley 09/26/06
Opal derives from the Latin opalus, meaning precious stone. Opal is an amorphous (i.e. noncrystalline), hydrated silicon dioxide (SiO2 * nH2O), that contains about 3-15% H2O by weight. The hardness of opal depends on its water content, and varies from 5.5-6.5, Water content can be quite variable, even within a single piece. The relatively high water content of most opal leaves it susceptible to damage by rapid or extreme changes in temperature. Opal mined from a damp or wet source, or that mined from volcanic host rock is more prone to dehydration than that mined from a dry source or from a sedimentary host rock. Dehydration may result in cracking (referred to crazing or checking) that greatly diminishes its value. The two major classes of opal are fire opal & common opal. A wide variety of names has been, and continues to be used, to describe various types of opal. Most have their basis in three important attributes; 1) body color; 2) transparency; 3) character of play-of-color and colors present. The following names are in common use:
- Black Opal- black, dark green, dark brown or other dark body color with vivid play-of-color. Often synonymous with " Australian Opal" or "Coal Black".
- Crystal Black Opal - Black opal that is transparent to translucent.
- Crystal Opal - transparent to translucent, having no body color but strong play of color.
- White Opal - translucent to opaque, with white body color and play-of-color. Once referred to as "Hungarian Opal".
- Boulder Opal - opal with play-of-color that is present in a matrix of dark brown or black sandstone (ironstone).
- Transparent or Semitransparent Opal - transparent opal with slight to no play-of-color with a body color that is yellow, orange, brown, red or colorless. Colorless called "Water Opal", "Jelly Opal", "Hyalite", "Contra-luz", "Hydrophane". Yellow, orange and red body color referred to sometimes as "Fire Opal" or "Cherry Opal". This material is commonly faceted rather than cabachoned.
- Common Opal ("Potch") - translucent to opaque, having nearly any body color, but no play-of-color. Known by a myriad of names; e.g. Matrix Opal, "Geyserite", "Wood Opal", etc.. "Common Opal" contrasts with "Precious Opal" in the categories above.
Fire Opal is unique among all gems because of its source of color (diffraction). The source of the play-of-color in fire opal is the semi periodic arrangement of the spheres of hydrated SiO2, whose centers are spaced at about the wavelength of visible light. These serve as a diffraction source for refracted and reflected light. Finer quality opal (that exhibiting great play-of-color) generally has more H2O than lesser quality material. Commercial deposits of Fire Opal have been found in Arizona, at the Spencer Mine in Idaho, Louisiana, the Virgin Valley in Nevada, and at Opal Butte in Oregon.
Common Opal, which has a random arrangement of SiO2 spheres, has no diffraction play of colors. However, common opal can be attractively colored yellow, brown, green, blue or pink by other trace elements or minerals. Common opal often forms petrified wood and is found in all of the western states.
Formation of Opal: Opal is formed at low temperatures (<200oC) by the precipitation of colloidal silica from groundwater moving through siliceous rocks in arid regions. Colloidal silica can derive from the following:
- Alteration of siliceous sedimentary rock (quartz sandstones) along cracks and cavities to deposit opal crusts or nodules - Australia
- Alteration of siliceous rhyolite volcanic rocks – Brazil, Mexico, Idaho
- Concentration of silica gel derived from marine organism in evaporating sea water; replacement of skeletal remains and shells by opal - Australia
Blue Grouse Opal Site
The Blue Grouse Opal Site, claimed by BGMS, is 99.99999% common blue or white opal occurring in fractures in porphyritic rhyolite flows. The visual phenocryst crystals in the rock are biotite. Occasionally, specimens have been found with pin point size flashes of fire opal. Rumor states that many years ago, the original claim holder found a fist size specimen full of fire. However, no one in our club has ever seen it. Jo Farley says that she has seen a couple of marble size specimens of fire opal that the Kaisers from Salmon, Idaho have found on the Blue Grouse Claim.
On Sept. 8, 2006, the BGMS hired an excavator and operator to dig on two areas of the Blue Grouse Claim. Ralph Luther spent the day at the Claim working with the excavator operator. Several people from the BGMS club searched the diggings for fire opal on Sept. 9th, 10th, and 11th. Jo Farley has the list of participants. On the 9th, the diggers were Robie Flynn, Bonnie Klein, Gayla Kaiser, Wes Williams, Jerry & Janice Sommers, Jo & Don Farley, and Ralph Luther. On the 10th, the diggers were Janet & Tim Huntley, former members Jim & Karen Rayner, Jo & Don Farley, and Penny & Ralph Luther. On the 11th, the diggers were Larry Jones, Harvey Sharp, Anna & Marvin Horner from Salmon, Jerry & Janice Sommers, Joe & Gayla Kaiser, Don & Jo Farley, Ralph Luther, and Wayne & Dan Farley. I, Wayne Farley, and my son Daniel searched on Sept. 11; during daylight for fire opal, and later that night with a long wave ultraviolet light for fluorescent specimens. We knew that the blue opal was fluorescent from previous years of collecting. I found a couple of pea size specimens of blue opal during the day search that had pin point flashes of color by examining numerous specimens with a 10 power hand lens. I then later examined these specimens at home with my 10 to 100 power microscope. One blue opal specimen had true green diffraction fire. The other specimen had a rainbow of colors from a fracture prism of opal. Samples of fire opal and fluorescent opal from the Blue Grouse and other areas were shown at the Sept. 26 club program.
Idaho Opal:
In Idaho, opal is the second largest contributor to the total value of gem material produced. The varieties produced include precious (white and pink), yellow, blue, pink, and common. The Spencer opal mine, the largest privately owned gem stone producer in the State, is the major producer of opal. At Spencer the precious opal occurs as one or more thin layers within common opal partially filling gas cavities within a rhyolite-obsidian flow. About 10% of the material is thick enough to cut into solid gems; the remainder is fashioned into doublets and triplets. The Spencer Mine is the source of pink common opal and pink precious opal. High silica volcanic ash deposits that derived from acid rock magma are believed to have been the source of the silica in Idaho’s opal deposits.
Differentiating Volcanics:
Acid magma produces Granite if cooled slowly, Rhyolite if cooled rapidly in surface flows, and Obsidian if cooled very rapidly. Basic flow rocks produce basalt, and are the chemical equivalent of Gabbro. Basic rocks are relatively low in silica and are not good sources for colloidal silica for producing opal. Volcanic rocks can be differentiated by their chemistry and physical characteristic as shown below:
Illustration by J. Johnson, USGS |
Basalt is the aphanites mineralogical equivalent of the coarse grained basic rock Gabbro. It is composed primarily of calcium plagioclase feldspar, pyroxene, minor olivine, and does not contain quartz or other light-color minerals. The overall color is dark grey to greenish black. It commonly has numerous bb to pea sized gas bubbles, which may be open or filled with secondary minerals. Basalt is never a source rock for opal, but rarely may be a recipient. Good examples of Basalt are the Columbian River Basalts in eastern Washington and Oregon, or Craters of the Moon in southern Idaho.
Rhyolite is the aphanites mineralogical equivalent of the acid rock Granite; and at times is porphyritic, meaning that it has scattered visual phenocryst crystals of quartz, sanidine (a potassium feldspar), biotite, and amphibole. Rhyolite colors vary from gray to reddish. The reddish tones are due to potassium feldspar porphyritic crystals in the rock. Rhyolites are generally found as flows, sills, and dykes, and occasionally as plugs. They never form widespread deposits because the viscous lava can only flow for short distances. Owing to their hardness and resistance to weathering, rhyolites often form jagged, angular, upstanding rock masses, and these features combined with their pale color make rhyolites relatively easy rocks to identify in the field. Acid volcanic rocks are more explosive than basic volcanic rocks, and rhyolite type rocks may also form as welded volcanic ash or welded volcanic breccia. These latter two are more susceptible to diagenesis of silicate minerals, and thus more favorable as a source for opal formation.
Fluorescent Opal:
Opal fluoresces in a variety of colors under ultra violet lights. Amongst the brightest is the clear botryoidal masses of Hyalite Opal from Spruce Pine, N. C. that fluoresce a bright green.
In Montana, about 25 miles south of Bozeman, fine Hyalite Opal and beautiful reddish-yellow translucent common opal occurs near the summit of Mount Blackmore, Gallatin County. This opal was discovered by geologist Albert Peale in 1872. One of the nearby peaks, Hyalite Peak (10,110 feet) has been named after the occurrence. The opal from this locality was once seriously considered for gem purposes and the name blackmorite proposed for it. A couple of pieces were for sale at one of the gem & mineral shows this summer.
I, Wayne Farley, also discovered this summer that common opal seams in the Crystal Park matrix fluoresces a bright green. I intend to follow this up next summer for good specimens. The fluorescent green in opal under the ultra violet light is due to a few PPM of uranium.
