Ruby and Sapphire: Aluminum-rich gems
"Don't tell fish stories where people know you; but particularly, don't tell them where they know the fish"
- Mark Twain
- Mark Twain
So, how do you know if you have a ruby or sapphire? Easy, if it is red or blue and attached to a 24-karat gold ring. Waza!
Ruby and sapphire are just different color versions of the mineral corundum: a simple aluminum-oxide (Al2O3), crystallized in trigonal symmetry under tremendous pressure and temperature. Just imagine an entire mountain sitting on a piece of aluminum foil. So, the majority of these gems form in metamorphic (recrystallized rock) environments, with others found in skarn (thermally metamorphosed marble), and others in rare, silica-poor, aluminum-rich, igneous rocks known as lamprophyre or syenite. Being aluminum oxide, the mineral is naturally transparent. But coloring agents give a ruby its red color and sapphire its blue, green, yellow, pink color.
When you find these incredible minerals, they form mostly hexagonal (six-sided) crystals, that may be a thin wafer, or a long prism. The gems are very hard (but brittle)! On Moh's hardness, they rate as 9, just below that of diamond. So, unless you have a diamond with you, they are not going to scratch. The gem has no cleavage according to most mineralogists, but instead has parting in three directions on which twinned crystals may grow. They are moderately heavy (specific gravity of 3.95 to 4.1), so you can pan them with a gold pan. Use a hand-lens and look at the basal termination (referred to as pinacoid) and in many specimens, you may see prominent, parallel lines, in two directions - these may look like cleavage, but are known as twin planes. And if you find your gems in Wyoming by reading my Gemstone book, the gem may be surrounded by a green reaction rim of zoisite. Such reaction rims are the result of the gemstone being out of equilibrium sometime in the geological past, and reacting with metamorphic fluids replacing portions of the corundum by zoisite, a complex alumno-silcate (Ca2Al3(SiO4)(Si2O7)O(OH). So, now you are sure you have a very nice ruby, but want a second opinion, send it to me, and if you never see it again, it was likely a valuable ruby.
Ruby and sapphire 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 corundum has beautiful color and/or transparency and yield ruby and sapphire gemstones. Thus when prospecting for these remember you are searching for rocks that are aluminum-rich and silica-poor, such as mica schist, serpentinite, aluminum-rich lamprophyre, anorthosite complexes, vermiculite schist, syenite, and marble (skarn). After a very tall mountain sat on those aluminum-rich rocks, then you would need to somehow get the deeply, buried, rocks to the earth's surface, during in an uplift (orogeny) along a fault. So, just get a general book on the geology of your area, start looking for places with the right kind of rocks. In Califormia, Oregon, Washington, British Columbia, and Alaska, there are many serpentines that are prime suspects for sapphire. If you live in the Appalachians in the eastern US, look for serpentine or vermiculite for rubies and sapphires. Then there is Michigan and Montana - keep you eyes open for mica schist, vermiculite and lamprophyre. Everywhere else in the US, look for the gems in a placer or your local jeweler. They may be hard to find in a placer because of those darn old twin planes which easily break due to abrasion during stream transport. So, look mountainous terrains and continental cores known as cratons that have extensive regions of metamorphic rock.
One host rock that often contains ruby and sapphire is vermiculite schist. 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, alone. Vermiculite is a aluminum-rich mineral that was valuable for fireproof insulation in the 1930s and 1940s. So check your local state or provincial geological survey offices for early to mid-20th century reports on geological studies related to vermiculite. Then go out and visit the sites and you may find some ruby, pink and white sapphire in the schist. Or you might find a variety of different colored sapphires. 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 to northern California, Oregon and Washington. Most represent segments of oceanic floor compressed and thrust onto the continental surface in the geological past because of plate collisions. Such serpentines have been subjected to considerable pressure and temperature during these tectonic processes, and some coastal serpentines may have benitoite, rare chromian diopside, pyrope garnet and even possibly 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 skarns. Skarns are carbonates-rich rocks altered at depth by aluminum-rich igneous rocks or metamorphic (metasomatic) processes. Most ruby skarns occur in central and southeast Asia, i.e., Vietnam, Sri Lanka, Afghanistan, Pakistan Azad-Kasmir, Tajikistan, Nepal, Myanmar (Burma) and southern China. Some of these are associated with marble in the Himalaya fold belt.
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) (the guy who bought this obviously had more money than sense). 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 and is found as barrel-shaped hexagonal (6-sided) prisms with rough, rounded, surfaces, and it often exhibits parting. Because of good rhombohedral & basal parting, corundum prisms often terminate at basal pinicoids (flat surfaces) that display striations due to repeated twinning.
Corundum comes in a variety of colors including gray, grayish green, blue, pink, brown, red & purple. Ruby is a deep pigeon’s-blood red translucent to transparent variety with adamantine luster: sapphire includes all other colors. Corundum displays a striking adamantine to vitreous luster noticeable in faceted gems. Its high specific gravity allows it will concentrate in black sands in placers. During sampling in Wyoming's central Laramie Range, a WGS research group recovered tiny rubies and sapphires in several pan-sample concentrates, suggesting that several corundum deposits remain to be discovered (Hausel and others, 1988).
THE CORUNDUM GEMSTONES
Cornflower Blue Sapphire
Light bluish-green Oriental Aquamarine
Green Oriental Emerald
Yellow-Green Oriental Chrysolite
Yellow Oriental Topaz
Aurora Red Oriental Hyacinth
Violet Oriental Amethyst
Corundum is often found in pelitic schist with a variety of alumino-silicate porphyroblasts (large crystals in a matrix of smaller crystals) such as mica, kyanite, garnet, sillimanite, andalusite, vermiculite & cordierite. Vermiculite schist is considered an alteration product of a former metapelite in which metapelite was desilicated to a mica-rich rock.
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. The deposit consists of corundum gneiss & schist in a 5,000-foot strike length with widths of 20 to 50 feet. The rock has 1 to 10% corundum encased in zoisite-mica schist.
Partially replaced specimens provide evidence of rubies of more than five inches in length & more than 2 inches in diameter. Nearby serpentinite west of the ruby schist has 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 associated with vermiculite schist west of Wheatland in Palmer Canyon. This deposit (N/2 Section 18, T24N, R70W) also has gem kyanite and cordierite, along with some 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 limiting the size of facetable material. Even so, significant amounts have good color (personal field notes, 1997). Small amounts of corundum have also been reported at the Grizzly Creek iolite (cordierite) deposit to the south & other localities to the north (Hausel & Sutherland, 2000).
Other corundum was identified in vermiculite schist in the Platte River Valley between the Medicine Bow and Sierra Madre Mountains. Another notable 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 primary source of this corundum remains undiscovered. Some nearby ruby schist float was found with gem-quality ruby (B.F. Frost, Personal communication).
See you on the outcrop - the GemHunter.