Ruby and Sapphire: Aluminum-rich gems

Ruby and sapphire (known to mineralogists as corundum) are 'aluminum oxides' that have been subjected to very high-pressure and temperature in the geological past equivalent to what geologists refer to as 'amphibolite facies metamorphism'. Sometimes the corundum has beautiful color, other times it may have good transparency to yield ruby and sapphire gemstones


Thus when prospecting for these gemstones remember you are searching for rocks that are aluminum rich - such as mica schist, serpentinite, aluminum-rich lamprophyre, anorthosites, vermiculite schists and marbles (corundum skarn) that have had considerable pressure applied to the rocks in the geological past. You would need much more than a hoard of overweight Walmart shoppers in electric carts to produce the necessary pressure - actually, a very high mountain range would be helpful to sit on the aluminum rich rocks for some time. Then you would need those buried rocks to be uplifted to the earth’s surface to get at them.

 

Such aluminum-rich rocks need to have been subjected to amphibolite grade metamorphism (high pressure and temperature). Thus you should be looking mountainous terrains or continental cores known as cratons that have extensive regions of metamorphic rocks, some containing amphibole and mica. 


Take for instance, most mountains in Wyoming and Montana (exclude places like the Absaroka Mountains around Yellowstone not only because the UN wants that region for their playground, but also because it is an igneous complex rather than metamorphic and not a good place to search for ruby or sapphire. Besides the Absaroka Mountains has too many unfriendly bears who would like to eat prospectors and hikers, and the area has too much copper, zinc, lead, manganese, gold and silver.


One host rock that contains ruby and sapphire is vermiculite schist. And simply by searching for vermiculite schist (referred to as glimmerite in some places in the world - such as Russia) I discovered six previously unreported ruby and sapphire deposits in Wyoming. Vermiculite is a aluminum-rich mineral that at one time was valuable for fireproof insulation in the 1930s and 1940s. So check your local state or provincial geological survey for early to mid-20th century reports on geological studies related to vermiculite. Then go out and visit the sites and you will likely find some ruby, pink and white sapphire in the schist.


Not all vermiculite deposits have corundum, but a significant number do (maybe 15 or 20%). Another rock that sometimes contains ruby and sapphire is serpentinite. Serpentinites are found in Montana and Wyoming as well as along the western coastal ranges of the US. In California, slabs of serpentine are found in the Mother Lode area that continue into northern California, Oregon and Washington. Most represent segments of the ocean floor that was compressed and thrust to the surface, while others may represent mantle material obducted onto the surface along a subduction zones related to continental collision. This means, such serpentines will have been subjected to considerable pressure and temperature during the tectonic processes. 


While exploring California for in situ diamonds, I found some extraordinary benitoite gems associated with serpentines as well as chromian diopside in other serpentines. With a little effort, I suspect many other gems could be found in these serpentines including benitoite, sapphire, chrome diopside, pyrope garnet and even diamond.


Another host rock for corundum is lamprophyre (an aluminum-rich igneous rock). Montana is known for sapphire-bearing lamprophyre. Some of the better source rocks for ruby are known as skarns. Skarns are carbonates-rich rocks altered at depth by aluminum-rich igneous rocks or metamorphic (metasomatic) processes to produce rubies with other gemstones. Most ruby skarns have been identified in central and southeast Asia including Vietnam, Sri Lanka, Afghanistan, Pakistan Azad-Kasmir, Tajikistan, Nepal, Myanmar (Burma) and southern China. Many of these are associated with mineralized marble in the Himalaya fold belt.


So how do you recognize a ruby in the raw? 


Ruby and sapphire form hexagonal prisms often with distinct parting planes that look like cleavage (no not that kind of cleavage - the kind you find in minerals). Sometimes they will exhibit triangular growth on the pinacoid surface of the crystal, or diamond-shaped parting. The six-sided (hexagonal) prism can be short wafer like or long barrel like crystal. Many also show evidence of replacement by green zoisite.

 

Rubies and sapphires bring visions of incredible value - but possibly none more than a Burmese ruby of 15.97 carats that sold at a Sotheby’s auction for US$3.63 million ($227,301/carat). More recently in 2005, Christie’s of New York sold a near perfect 8.01-carat Burmese ruby with strong fluorescence for US$2.2 million at a record price for a ruby (US$274,656/carat)! And a 62-carat royal blue rectangular cut sapphire was purchased for $2.8 million ($45,000/carat). Rubies are more valuable than sapphires, even though the are the same mineral.

 

Corundum is the second hardest naturally occurring mineral: only diamond is harder. Corundum is found as barrel-shaped hexagonal (6-sided) prisms with rough, rounded, surfaces often exhibiting distinct parting. Because of good rhombohedral & basal parting corundum prisms often terminate at basal pinicoids (flat surfaces) & display striations due to repeated twinning.


Corundum comes in a variety of colors including gray, grayish green, blue, pink, brown, red & purple. Some is used to produce extraordinary gemstones. Ruby is the deep pigeon’s-blood red translucent to transparent variety of corundum with adamantine luster & sapphire includes all other colors. Corundum displays a striking adamantine to vitreous luster noticeable in faceted gemstones. With high specific gravity (4 to 4.1) it will concentrate in black sands in streams, where it can be panned with a gold pan (similar to gold). During sampling in the central Laramie Range of Wyoming, my research group recovered tiny rubies and sapphires in several sample concentrates, suggesting that several corundum deposits remain to be discovered (get a copy of that old report, and follow up the locations of the samples sites particularly since no one has to my knowledge - Hausel and others, 1988).


THE CORUNDUM GEMSTONES

COLOR                      VARIETY

Red                               Ruby

Cornflower                    Blue Sapphire

Colorless                      Leuco-sapphire

Light bluish-green        Oriental Aquamarine

Green                           Oriental Emerald

Yellow-Green                Oriental Chrysolite

Yellow                           Oriental Topaz

Aurora Red                   Oriental Hyacinth

Violet                            Oriental Amethyst

 

Corundum, a high-pressure aluminum oxide, is found with silica-poor aluminum-rich metamorphic rocks in areas with abundant metapelite (mica-rich rocks). Such metapelite may contain a variety of alumino-silicate porphyroblasts (larger crystals in a matrix of smaller crystals) such as mica, kyanite, garnet, sillimanite, andalusite, vermiculite & cordierite. The corundum itself is typically found in vermiculite schist & aluminum-rich serpentinites. Vermiculite schist is considered an alteration product of a former metapelite in which metapelite was desilicated leaving mica-rich rock known as vermiculite schist or glimmerite schist. After I realized there was a close association of vermiculite with ruby, I discovered several ruby deposits in Wyoming. 


Corundum has been found at a number of places in Wyoming. One locality northwest of Jeffrey City, known as the Red Dwarf deposit (sections 13 and 24, T30N, R93W), was investigated by me several years ago. The deposit consists of corundum gneiss & schist. I mapped a 5,000 foot strike length with widths of 20 to 50 feet. The rock has 1 to 10% corundum porphyroblasts encased in zoisite-fuchsite reaction rims & considerable fuchsite (green mica) and zoisite pseudomorphs after corundum. Where found, some of corunudum is translucent with good color.


Corundum may be light purplish-pink, lavender, to reddish-purple, and range from millimeter size to more than two inches across. Some gem-quality corundum was found in the past & partially replaced specimens provide evidence for rubies of five inches (or more) in length & more than 2 inches in diameter in this area. Nearby serpentinite west of the ruby schist contains tiny (millimeter size), light-blue, translucent to opaque corundum. Locally, the serpentinite has 20 to 40% corundum.

 

At another deposit known as the Abernathy deposit (section 26, T30N, R96W) near Sweetwater Station, pale-blue and white corundum is found in mica schist. The corundum is abundant and occurs as one-inch diameter nodules in the schist.


Corundum is also associated with vermiculite schist (glimmerite) west of Wheatland in Palmer Canyon. This deposit (N/2 Section 18, T24N, R70W) also has gem kyanite, cordierite & sillimanite schist & gneiss. The corundum forms small, hexagonal, pink, red & white grains from about 0.1 to 0.3 inch across. Many grains have well-developed parting which limits the size of facetable material. Even so, significant percentages have excellent color & are transparent to translucent (personal field notes, 1997). Locally, the schist may contain >20% corundum. Small amounts of corundum have also been identified at the Grizzly Creek iolite (cordierite) deposit to the south & other localities to the north (Hausel & Sutherland, 2000).


Some corundum was identified in vermiculite schist in the Platte River Valley between the Medicine Bow and Sierra Madre Mountains. Another notable corundum locality is the Big Sandy opening along the southern margin of the Wind River Mountains, where hundreds of corundum crystals weighing up to 90 carats were collected from Squaw Creek by prospectors (Russ and Joe Sims). The source of this corundum remains undiscovered. Some nearby ruby schist float was found with gem-quality ruby (B.F. Frost, Personal communication) suggesting that a potentially significant ruby deposit awaits discovery. 


 HOW TO FIND RUBY & SAPPHIRE


To find ruby & sapphire (and other gemstones), look for areas with abundant mica schists or rare lamprophyres!  Get familiar with the characteristics of the mineral (see my 2014 Gem Book). In the rough, ruby & sapphire (corundum) form rod-like prisms much of the time. They are often found in metamorphic rocks. They are sometimes found in skarns (altered limestone) and rarely in lamprophyres. How I found ruby deposits in Wyoming is I paid attention to the mineral habit (not all geologists can recognize ruby, in fact, we had one geologist at the Wyoming Geological Survey who picked up a rock with about 20% ruby but misidentified it as garnet). He was a very good geologist, but just made a mistake on identification which is easy to do. After I realized there was a close association with ruby & vermiculite, all I did was go get a copy of an old report on Vermiculite Deposits of Wyoming & started visiting each site (until I was also harassed) & discovered ruby in about 30% of these deposits!  So search your local literature for corundum, vermiculite, skarns and lamprophyres and soon you may become a ruby & sapphire barren!


See you on the outcrop - the GemHunter.