Corundum is a naturally occurring mineral defined by its chemical composition as aluminum oxide (\(Al_2O_3\)). It is renowned for its exceptional durability, ranking at 9 on the Mohs scale of hardness, second only to diamond. While corundum is utilized widely as an industrial abrasive, its fame rests on its gem-quality varieties. These transparent crystals are known globally as ruby when red, and sapphire when blue or any other color.
Defining the Necessary Chemical Environment
The formation of corundum is fundamentally a chemical balancing act governed by the availability of elements within the Earth’s crust. Its formula, \(Al_2O_3\), reveals that it requires an environment rich in aluminum. However, the presence of aluminum alone is not sufficient for corundum to crystallize.
The primary constraint on corundum formation is the near-absence of silica (\(SiO_2\)). Silica is the most abundant oxide in the crust, and aluminum has a strong tendency to bond with it, forming common silicate minerals like feldspar and mica. If significant silica is present, aluminum atoms will be consumed in the creation of these silicate compounds. Therefore, corundum can only form in silica-undersaturated environments, ensuring that the aluminum bonds only with oxygen.
Primary Corundum Formation in Crustal Rocks
The initial crystallization of corundum occurs deep within the Earth’s crust under high heat and pressure. This primary formation is divided into two main geological settings: metamorphic and igneous. These pathways represent distinct mechanisms for achieving the necessary aluminum-rich, silica-poor chemical environment.
Metamorphic Environments
Corundum forms through the intense alteration of existing sedimentary rocks that were already rich in aluminum, such as bauxite or aluminous shale. Extreme heat and pressure, often associated with regional metamorphism during mountain-building events, drive out water and other volatile components. This process concentrates the aluminum and transforms the parent rock into corundum-bearing rocks like schist, gneiss, or marble.
Igneous Environments
Igneous formation occurs when corundum crystallizes directly from a magma melt that is naturally low in silica. Examples include alkaline, silica-poor intrusive rocks like nepheline syenites. In these magmas, excess aluminum is not fully incorporated into silicates, allowing it to bond with oxygen and form corundum crystals. Corundum can also form in basaltic volcanic rocks, where crystals are brought up rapidly from the mantle as xenocrysts.
The Role of Weathering in Creating Gem Deposits
While corundum is born deep within the crust, most commercially viable gem deposits are not mined directly from the primary host rock. Instead, they are found in secondary deposits created through weathering and erosion. Corundum’s hardness, ranking 9 on the Mohs scale, gives it an advantage over nearly all other rock-forming minerals.
As the corundum-bearing host rocks are uplifted and exposed, they are gradually broken down by wind, water, and ice. The surrounding, softer minerals, such as feldspars and micas, weather away much faster than the corundum crystals. Water transports the liberated crystals downhill, often over vast distances.
Gravity and water flow act as natural concentrators, sorting the heavier corundum crystals from the lighter mineral fragments. These crystals settle in riverbeds, alluvial fans, and ancient terraces, forming what are known as placer or alluvial deposits. The majority of the world’s finest rubies and sapphires are recovered from these secondary gem gravels.
How Trace Elements Determine Color and Gem Quality
Pure corundum, composed solely of aluminum and oxygen, is naturally colorless, often referred to as white sapphire. The range of colors seen in rubies and sapphires is determined by specific trace elements incorporated into the crystal lattice. These elements substitute for a small fraction of the aluminum atoms, altering how the crystal absorbs and transmits light.
The red color of a ruby is caused by the presence of chromium, where chromium ions (\(Cr^{3+}\)) replace some of the aluminum ions in the crystal structure. The concentration of this element must be substantial enough to produce the saturated red hue. Blue sapphire owes its color to a combination of iron and titanium, which interact within the crystal structure to create the deep blue coloration.
Other colors, collectively known as fancy sapphires, are caused by varying trace element combinations. Yellow and green sapphires are colored by iron alone, while vanadium can produce purple or color-change effects. The chemical environment of the parent rock dictates which trace elements are available to be captured, determining the final color and quality of the resulting gem.