Ruby is a mineral, the red gem variety of corundum. Corundum is an aluminum oxide compound that crystallizes under specific conditions of high pressure and temperature. Its formation requires aluminum-rich source material and a distinct lack of silica, a common mineral component in the Earth’s crust. The geological environment determines the type of host rock in which ruby is found, typically altered metamorphic rock or, less commonly, igneous rock.
The Mineralogical Identity of Ruby
Ruby is chemically defined as aluminum oxide, the pure composition of the mineral corundum. This mineral is a rock-forming constituent distinguished by its dense, trigonal crystal structure. The tight arrangement of aluminum and oxygen atoms gives corundum its remarkable physical durability.
This crystal lattice is responsible for the mineral’s hardness, rated as 9 on the Mohs scale, making it the second hardest naturally occurring mineral after diamond. The stability of the corundum structure means it resists chemical weathering and physical abrasion. Pure corundum is naturally colorless; trace elements transform it into the colored gem varieties.
Corundum also possesses high density, with a specific gravity typically ranging from 3.95 to 4.1. This combination of extreme hardness and high density allows the mineral to survive erosion and concentrate in secondary deposits.
The Origin of Ruby’s Red Color
The vibrant red hue that defines a ruby is the result of a trace element impurity, chromium, which substitutes for aluminum ions within the crystal structure during formation.
The presence of chromium distinguishes ruby from colorless corundum and sapphire. All other colors of gem-quality corundum—including blue, yellow, pink, and green—are classified as sapphire. The chromium atoms absorb light in the yellow-green part of the spectrum, transmitting the fiery red color.
This color mechanism is enhanced by fluorescence, also caused by the chromium ions. Chromium absorbs ultraviolet light and re-emits it as visible red light, which adds to the intensity and glow. Rubies with a low iron content, often those found in marble, typically exhibit stronger fluorescence, creating a more vivid, pure crimson hue.
Geological Environments and Formation
Rubies are found in primary deposits within two main types of host rock: metamorphic and igneous. The most significant and highly prized rubies, such as those from Myanmar, are typically found embedded in metamorphic rocks like marble. Marble is formed when limestone, a calcium carbonate rock, undergoes regional metamorphism involving intense heat and pressure.
This metamorphic environment, often depleted in iron, allows the chromium to express the pure red color without interference. Other aluminum-rich metamorphic rocks like gneiss and schist can also host ruby deposits. These deposits are often associated with less vibrant pinkish-red to deep red shades.
Less commonly, rubies can be found in silica-poor igneous rocks, such as basalt or alkaline syenite. These basalt-hosted rubies, found in locations like Thailand and Cambodia, tend to have a higher iron content due to the surrounding rock chemistry. The iron acts as a color-masking agent, often resulting in a darker, more brownish-red appearance with less natural red fluorescence.
The majority of commercially mined rubies are not extracted directly from the primary host rock but from secondary, or alluvial, deposits. As the original host rock erodes over millions of years, the durable ruby crystals are released. They are then transported and deposited by water in riverbeds or alluvial fans, where their high specific gravity allows them to concentrate alongside other heavy minerals.