The question of whether true rubies exist in the Ruby Mountains of Nevada is compelling, driven entirely by the range’s suggestive name. This prominent feature of the Basin and Range Province in northeastern Nevada naturally sparks curiosity about the presence of valuable gemstones. The answer lies not in finding corundum, but in a classic case of mineral misidentification by early prospectors. The historical naming of the Ruby Mountains is a testament to the vibrant red color of a much more common mineral found abundantly in the region.
The Truth Behind the Name
There are no economically viable deposits of true rubies in the Ruby Mountains; the name is a geological misnomer. Early explorers and soldiers in the 1800s, often panning stream beds for gold, noticed an abundance of small, deep red crystals. They mistakenly identified these bright red stones as rubies, which are a variety of the mineral corundum, leading to the name “Ruby Mountains.”
The red stones causing this confusion were actually various types of garnet, primarily the iron-aluminum silicate known as almandine. Garnets are silicate minerals that form readily under the metamorphic conditions prevalent in the mountains, explaining their high concentration. The primary difference between the two red gems is their chemical composition and hardness: rubies are aluminum oxide, while garnets are silicates.
Rubies possess a Mohs hardness of 9, making them durable, second only to diamond. Garnets are significantly softer, typically ranging from 6.5 to 7.5 on the same scale. Although the red garnets share a similar hue to rubies, they lack the intense color, high refractive index, and internal glow that characterize the much rarer corundum.
Geological Identity of the Ruby Mountains
The Ruby Mountains are a classic example of a metamorphic core complex, a geological feature where deep crustal rock is brought to the surface through extensional forces. The rocks in the central core are highly metamorphosed, consisting predominantly of schists, gneisses, and quartzite. This complex geological structure formed during a period of crustal extension following an earlier phase of compression, resulting in the exhumation of rocks originally buried up to 22 miles deep.
The high temperatures and pressures associated with this metamorphic process were ideal for forming silicate minerals. Minerals like almandine garnet, staurolite, and kyanite are common byproducts of the intense metamorphism of aluminum-rich sedimentary rocks, such as shales. Garnets are stable at the amphibolite facies conditions that affected much of the range.
The presence of these silica-rich metamorphic rocks fundamentally explains the abundance of garnets and the absence of rubies. The chemical environment, dominated by silica, is incompatible with the formation of corundum, which requires an environment high in aluminum but extremely low in silica.
The Formation Conditions for True Rubies
True rubies are a variety of corundum, which is crystalline aluminum oxide (Al2O3). Their distinct red color comes from trace amounts of chromium replacing some aluminum atoms in the crystal structure. Their formation is an intricate process requiring a specific and rare combination of chemical and physical conditions.
Rubies primarily form in two distinct geological settings: metamorphic rocks like marble or gneiss, and certain igneous rocks such as basalt. In both environments, the host rock must be depleted in silicon dioxide (SiO2) and significantly enriched in aluminum.
When rubies form in metamorphic environments, they often grow in marble where the source rock was a silica-poor limestone. Igneous formation typically occurs in silica-poor rocks like syenite or is associated with alkali basaltic volcanism. The formation requires intense heat and pressure, often found in mountain-building zones or deep within the Earth’s crust. This need for high aluminum and low silica is the main reason rubies are globally rare and why they do not form in the silica-rich metamorphic core complex of the Ruby Mountains.