Iolite - An Extraordinary Gem
Iolite is the gemologist term for the mineral cordierite. It is a magnesium-iron-aluminum-silicate with iron present in solid solution with magnesium, such that there exists both magnesium-rich cordierite and iron-rich cordierite (refereed to as sekaninaite). Thus, the chemical formulas for end members of the solid solution series are (Mg,Fe)2Al3(Si5AlO18) and (Fe,Mg)2Al3(Si5AlO18). The mineral was named after a French geologist by the name of Louis Cordier in 1813.
Good quality cordierite is often used for gems. The name "iolite" comes from the Greek word for violet. It is also referred to as dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong pleochroism. It has also been called "water-sapphire" and "Vikings Compass" because of its usefulness in determining the location of the sun on overcast days, the Vikings apparently used it for this purpose. When prospecting, iolite is difficult to find with a gold pan unless you are panning near the source. The best way to prospect for iolite is to search for metamorphosed aluminum-rich rocks such as biolite schist and gneiss.
Gem quality iolite varies in color from sapphire blue to blue violet to yellowish gray to light blue as the light angle changes. It is softer than sapphire, orthorhombic and has massive to pseudo-hexagonal crystal habit. It is described in Australia (Northern Territory), Brazil, Burma, Canada (Northwest Territories), Finland, Germany, Greenland, India, Madagascar, Myanmar, Mozambique, Namibia, Norway, Sri Lanka, Tanzania, United States (Connecticut and Wyoming) and Zimbabwe. The largest known iolite crystal weighed more than 24,000 carats, and was discovered in Wyoming. But sitting in outcrop, too big to recover by one geologist, is a gemstone of potentially, tens of millions of carats - the gem is the size of a Smart car!
Iolite was unknown in Wyoming prior to it being found in the mid-1990s following discovery of a fabulous gem-quality iolite occurrence at Palmer Canyon west of Wheatland, Wyoming. The only other report of iolite in Wyoming at that time was by John Sinkankas in his gemstone book that alluded to a giant, rich, iolite deposit, but gave no specifics, no location, and left one wondering where this deposit was located. Even to this day, we do not know what deposit or where Sinkankas was referring to. Likely, he was referring to Sherman Hills-Ragged Top Mountain in the Laramie Mountains? Or was describing a altogether different deposit in Wyoming or even in another state such as Montana? I contacted Sinkankas before he died to ask about the reference: he could not remember anything about the deposit, or where his information cam from - but I suspect it was related to the Sherman Mountain deposit.
After the Palmer Canyon deposit, I came across three other iolite deposits in the same region of the central Laramie Mountains and predicted where other iolite deposits would be found in Wyoming. Two of the deposits I came across after Palmer Canyon are likely world-class multi-gem deposits containing iolite. The Grizzly Creek deposit is certainly world class, but the Sherman Hills-Ragged Top remains uncertain, but likely is an incredible deposit. I had hoped to investigate this latter deposit, but unfortunately ran into a roadblock (i.e., corrupt Wyoming Geological Survey director and corrupt governor) and had to leave the deposit for someone else to investigate in the future, even though I found some very high-quality gem iolite at the periphery of the deposit indicating the deposit could host considerable gem material. In fact, along the periphery, all I found was high-quality gem material!!! Based on mapping, sampling and trenching prior to 1949, the tonnage of cordierite reported for this deposit would equal 2.5 trillion carats of gemstones!!!
Giant iolite gemstones including the largest iolites in the world were discovered in the Laramie Mountains of Wyoming. Gemstones weighing >24,000 carats were recovered from Grizzly Creek with masses left in the outcrop that are estimated to include stones of more than a hundred thousand carats and some that could potentially top tens of millions of carats. Many of these were massive transparent stones and others were highly fractured but also transparent with excellent color.
Iolite deposits are now known at Palmer Canyon, Grizzly Creek, Sherman and Ragged Top Mountains, and Owen Creek in the Laramie Mountains. Based on favorable geology (Hausel and Sutherland, 2000), there are likely many other iolite gems waiting to be found in this region. These discoveries represent the largest iolite gem deposits and individual iolite gemstones found anywhere on earth. Yet, these remained unknown until the end of the 20th century and start of 21st century (Hausel, 2005b).
What is Iolite?
Gemologists refer to gem-quality cordierite as iolite. Iolite is also referred to as water sapphire. Geologists & mineralogists refer to iolite as cordierite, and in the past, the mineral has also been termed dichroite.
Cordierite, typically is found in the vicinity of other alumino-silicates, such as andalusite, kyanite & sillimanite, within amphibolite-grade metamorphic complexes. Host rocks include alumina-rich mica schist (metapelite). Cordierite also occurs as a replacement mineral in alumina-rich, syenite-anorthosite complexes & in shale. Since the Laramie Range is intruded by an enormous syenite-anorthosite complex with many labradorite (spectrolite) gemstones, this batholith needs to be explored for iolite!
Iolite may form short, prismatic, crystals with rectangular cross-sections as well as granular masses and nodules of various shades of blue, bluish-violet, gray, and brown. Fresh cordierite has a hardness of 7 and specific gravity of 2.55 to 2.75. The hardness is favorable for durable gems. The principal deposits that supply much of the world’s market are Sri Lanka placers in spite of low specific gravity of the mineral (i.e., it is not favorably concentrated in placers).
Iolite exhibits strong pleochroism that varies from light gray, dark violet-blue, to light sapphire blue. The gem may appear deepest blue when viewed down the c-axis of the mineral, and light blue to light grey in other orientations. These color variations are one of the attractive characteristics of this gem. Iolite may be enclosed by a reaction rim of pinite (basically secondary muscovite or sericite mica), or stained by limonite (Hausel, 2002).
Iolite is a beautiful stone that rivals tanzanite. However, tanzanite is more valuable because of marketing strategies. Good iolite is every bit as attractive and wearable and should be valued similar to tanzanite, but it will take brilliant marketing strategies to promote this gemstone. The luster of iolite is vitreous and when polished, it increases its luster. Faceted iolite gems greater than 12-carats are unheard of on the world market, even so, rough material collected at Palmer Canyon and Grizzly Creek include numerous stones larger than 12-carats. Some larger Wyoming porphyroblasts weigh >100 carats, with others more than 1000 carats.
In Wyoming, cordierite has been found in gneiss with quartz and biotite as porphyroblasts with xenoblastic texture (Hausel, personal field notes, 1995). Worldwide, the highest quality gems are found as pebbles in Sri Lanka. With a good marketing plan, Wyoming could easily control the world market of iolite and potentially promote this gem and bring a new industry (mining, cutting, and marketing) to the Cowboy state, but it will take effort, work, and funding from the state legislature. Iolite currently sells for a trivial $15 to $30 a carat for loose gemstones compared to tanzanite which sells for around $100 per carat. But loaded in jewelry, tanzanite can sell for more than $1000/carat. A good marketing ploy with a good story and unique name, can bring high prices for iolite, as it did for tanzanite. Additionally, Wyoming has both magnesium- and iron-rich varieties of iolite that will produce an array of blue gems from light blue to dark blue to amethyst blue to black.
Large nodular masses of iolite (porphyoblasts) were discovered in two separate deposits in Archean (>2.5 billion year old) gneiss in Wyoming, and a giant, disseminated, deposit may occur in the Laramie Range anorthosite-syenite batholith (Hausel 2002; 2004; 2006a). These represent the largest iolite resources in the world (Hausel, 2005b).
Two deposits (Palmer Canyon and Grizzly Creek) are poly-gem occurrences with ruby & sapphire as accessory gems along with considerable kyanite (some gem-quality) in vermiculite and biotite schist and gneiss. Based on metamorphic grade, there is also possibility for other gems such as andalusite, sillimanite, and staurolite in these areas. Not far from the Sherman Hills deposit, are spectrolite (labradorite) gemstones. After these labradorites were described by Hausel (2014), Wyoming prospectors and rock hunters began collecting this material and produced many, beautiful gems! And it doesn't end there. We also found evidence for many additional ruby, sapphire, and diamond deposits in this region based on stream sediment samples. The metapelites in this area of the central Laramie Mountains represent enclaves of aluminous schist & gneiss. The Ragged Top and Sherman Mountain deposit is likely hosted by anorthositic-syenitic rocks (1.5 Ga).
Iolite was discovered in 1995 at Palmer Canyon during field reconnaissance (Hausel, 2002). The deposit lies along the eastern flank of the central Laramie Range, 16-miles west of Wheatland within Archean quartzofeldspathic gneiss, granite gneiss, pelitic schist, and biotite-chlorite-vermiculite schist north of the Elmers Rock greenstone belt. To visit this area, fly to 'Palmer Canyon Rd, Wheatland WY 82201' using Google Earth to place you about 4 miles west of the deposit. A shallow prospect pit was dug in vermiculite adjacent to the iolite deposit, prior to 1944. Only a small amount of corundum-vermiculite was found. The vermiculite contains chlorite, kyanite and corundum. Samples of vermiculite schist collected from a small prospect pit contained as much as 10 to 20% corundum. A minor amount of corundum is gem quality. Several specimens produced high-quality ruby and pink sapphire.
Cordierite was discovered in nearby quartzofeldspathic gneiss. Samples of cordierite gneiss yielded transparent cordierite grains including stones weighing >100 carats. Some gneiss from the property had as much as 20% transparent cordierite. The cordierite occurs as rounded to disseminated grains and large nodules. Foliation in the host parallels the margin of nodules and in some samples appears to terminate against nodular boundaries, suggesting some of the cordierite formed during a post regional metamorphic event. Nearby, kyanite schist has 20 to 50% excellent, light to sky blue, lesser tawny, green & red gem-quality kyanite prisms.
Transparent blue iolite was found as large porphyroblasts, nodules & disseminated grains in gneiss adjacent to corundum and kyanite schist. The iolite was traced over a strike length of 500 feet and continues under soil for an unknown distance. A handful of large nodules found at the time of discovery, include high-quality transparent gem known as the ‘Palmer Canyon Blue Star’ of 342.8 grams (1,714 carats), which was the largest iolite gemstone in the world at time of discovery. In addition to clear, transparent, violet blue gem-quality cordierite, black translucent cordierite (Palmer Canyon Black) was recovered. The Palmer Canyon Black produces attractive cabochons and faceted stones. Palmer Canyon Black gems of 4 to 6 carats were faceted in Sri Lanka. At least 10 different categories of gems were described at Palmer Canyon by the author.
Much high quality rough ranges from pleasing violet to a very light-blue color with only a hint of cleavage and parting. Microscopic examination shows a few mineral inclusions in some gems. Gray to dark gray cordierite has well-developed parting and cleavage. A group of cabochons were cut that weighed 0.27 to 3.02 carats. These are dark-gray to black, translucent to opaque, near gems with distinct cleavage, parting and fractures.
Two poor-quality specimens were faceted to yielded a 3.9-carat lozenge-cut stone and a 3.4-carat marquise. Both were flawed with visible cleavage and parting. However, both produced surprisingly attractive jewelry when mounted in necklaces. Some bluish gray to gray translucent to cloudy material represents rehealed mylonitic cordierite that is poor-quality
Grizzly Creek was accessed from the Palmer Canyon road about 4 miles east of the Palmer Canyon deposit at the base of the Laramie Range. To fly to this deposit, fly to 'Grizzly Creek, Rock River, WY' on Google Earth. Following discovery of Palmer Canyon, similar deposits were predicted to exist in Grizzly Creek (Hausel and Sutherland, 2000). The thermal metamorphic event responsible for the large cordierite porphyroblasts at Palmer Canyon appear to have been widespread in the central Laramie Range. The earlier prograde metamorphic event produced large prophyroblasts of kyanite in adjacent rock.
It became clear during the initial field investigation that a major gem deposit had been discovered. Very large masses of gem-quality iolite were found, as well as quantities of gem-grade kyanite. Cordierite at Grizzly Creek is surrounded by kyanite and sillimanite schists that have some minor corundum. The kyanite and sillimanite schist lies in a 300 by 5000 foot belt of metapelite. Much kyanite appears to be cabochon grade and has a very pleasing, sky-blue color, with some tawny and pink specimens.
The iolite is massive. This deposit may represent the largest iolite deposit in the world. During reconnaissance, specimens of massive iolite were collected including a football-size transparent gemstone that weighed 24,150-carats the largest in the world. It now resides in the Wyoming Geological Survey display. However, this stone is dwarfed by masses of material that remain in place in Grizzly Creek. Some of the massive gem material will require quarrying operations. It is very likely gem specimens >1 ton (>4.5 million carats) could be recovered! In outcrop, the iolite is weakly iron-stained and shows excellent light-blue color & transparency on fresh surfaces. It is not known how much if any of this material has been destroyed by mylonitization (deformation). For example, several specimens collected at Palmer Canyon showed distinct mylonitic to ultramylonitic texture in thin section that resulted in cloudy, light-blue and glassy material of poor quality.
Sherman Hills-Ragged Top
The first report of iolite in Wyoming was by Sinkankas (1959). A brief description indicated that iolite was a widespread constituent of schist & gneiss. In describing a deposit Sinkankas wrote, “one estimate has placed the quantity available at thousands of tons” (remember that a ton contains 4.5 million carats!!!). A couple of specimens I found along the southern edge of the Ragged Top cordierite deposit were glassy broken fragments of rather light blue color, verging towards grayish, small sections are clear and suitable for faceted gems.
Sinkankas also wrote ... "It is entirely possible that important amounts of gem quality material will be produced from this locality in the future.” Unfortunately, Sinkankas did not give a location: the whereabouts of this giant deposit remains unknown?
At the time of writing (1959), only one cordierite deposit had been described in the literature. The deposit, known as the Sherman Mountains deposit 15-miles south of Palmer Canyon. The deposit is in Proterozoic (1.4 Ga) metanorite, syenite and syenite-diorite gneiss of the Laramie anorthosite complex intrude the Cheyenne suture (1.8-1.6 Ga) zone. Widespread lenticular to tabular layers of cordierite is found in metanorite (hypersthene gneiss), gneiss and syenite along the southern margin of the anorthosite complex (1.5 Ga).
The host rock is described to have 50-80% cordierite. The occurrence lies 0.5-mile west of Ragged Top Mountain in a belt 0.3 to 1.2 miles (0.5-1.9 km) wide & 6 miles (9.6 km) long. The host gneiss is highly foliated, intensely folded and contorted. The weathered cordierite was described to have dark brown surfaces that yield to blue or bluish gray massive material on fresh surfaces. I was able to obtain small samples of disseminated cordierite along the margin of this deposit. All of the cordierite was very high-quality gem material. Massive portions of this deposit remain unevaluated for gems and may represent the largest, colored gem deposit in the world. This is likely the same deposit described by Sinkankas.
The deposit is described to be scattered over a few square miles in lenticular to tabular masses in low ridges of metanorite 5-miles-long and 0.25- to 1-mile-wide. Some exposures are described as having 60 to 80% cordierite. It was estimated that the combined deposits with strike lengths of 100-feet or more, have >453,600 tonnes of cordierite In other words, a potential resource of 2.27 trillion carats!
Sinkankas (personal communication, 2002) indicated that much of the material was gem-quality (Sinkankas, 1959, 1964). This (along with Grizzly Creek) could be one of the greatest discoveries of colored gemstones. To visit this deposit from your computer, fly to: 'Ragged Top Mountain, East Albany, WY 82072' on Google Earth. While you are in the area looking at the foliated anorthosite-syenite complex, note that iolites are scattered over this area and to the southwest is a distinct group of lakes that line up (structurally controlled) with what may be some carbonate salts in the soils. These are great candidates for undiscovered diamond deposits that are known as the Horse Creek Lakes cryptovolcanic structures.
Another iolite deposit in the northern Laramie Mountains is referred to as Owen Creek (Hausel, 2009). This contains kyanite, sillimanite, cordierite & relict staurolite in pelitic schist & remains unexplored. Cordierite is also reported at South Pass (Hausel, 1991), Copper Mountain (Hausel and others, 1985), in the Sierra Madre, and in the Powder River Basin. The cordierite occurrences at South Pass were investigated by me during field mapping of the greenstone belt. I did not observe any gem-quality material there. However, I highly recommend investigations of cordierite at Copper Mountain as this supracrustal belt contains abundant metapelite (alumina-rich rock) that was subjected to similar metamorphic conditions as the Elmer?s Rock greenstone belt. For more information, see Hausel (2009a, 2009b, 2009c).
How To Find Iolite
First you need to take me (just kidding). I love to hunt gemstones & I have a nose for finding iolite.
Iolite is found in amphibolite-grade metapelites (mica rich rocks). Typically it occurs where the rocks have been subjected to high pressures & temperatures that reached the alumino-silicate triple point (nearby rocks contain kyanite, sillimanite & andalusite). It is also found as a replacement mineral in some syenites and anorthosites. Few people know how to recognize this gem.
What I found is not only the largest gemstones, but also the largest colored gem deposits in the world. You would think that this would attract some interest. Visualize this. Iolite sells for about $25 to $125 per faceted stone. It can be faceted for about $0.50 a carat. Gems of iolite larger than 12 carats are unheard of on the world market. It doesn't take a nuclear physicist to see the potential here.
And there are likely more iolite deposits in this region. But someone has to look. And after billions of carats of the gem have already been identified, it will take the legislature with its deep pockets to finance research into marketing, mining and cutting the gem material in Wyoming.
* From a lecture by W. Dan Hausel to the Wyoming Geological Association in Casper.