Tourmaline is the name for a large group of complex crystalline borosilicate minerals that share a common crystal structure. Its chemical variability allows the crystal lattice to accommodate a wide range of elements, including iron, magnesium, lithium, sodium, and calcium. The presence and proportion of these trace elements dictate the mineral’s final properties, including its color and specific electrical behaviors. This variability allows for the existence of over 40 distinct tourmaline species.
Defining Physical Characteristics
Tourmaline crystals belong to the trigonal crystal system, often presenting as long, slender, or thick prismatic columns. A characteristic feature is their typically rounded, three-sided cross-section, which is unique among common minerals. The faces of these prisms are frequently marked by heavy vertical grooves called striations.
The mineral exhibits an impressive hardness, scoring between 7 and 7.5 on the Mohs scale, making it resistant to scratching and suitable for use in jewelry. It possesses an indistinct cleavage, meaning it tends to fracture unevenly rather than breaking along flat planes. Tourmaline generally displays a vitreous, or glassy, luster.
The specific gravity ranges from approximately 2.82 to 3.32, with the exact value dependent on the chemical composition. Iron-rich varieties, such as schorl, tend to have a higher specific gravity compared to lithium-rich varieties like elbaite. Tourmaline is famous for having the widest color range of any mineral group, appearing in virtually every color of the spectrum.
The color variation is a direct result of trace elements substituting into the crystal structure. Manganese often causes pink and red hues, while iron typically produces greens, blues, and black. This allows a single crystal to display multiple colors in distinct zones, often forming striking bi-colored or tri-colored specimens like “watermelon tourmaline.”
Distinctive Electrical and Optical Behaviors
The internal structure of tourmaline is asymmetrical, lacking a center of symmetry, which gives rise to its most scientifically interesting characteristics: pyroelectricity and piezoelectricity. Pyroelectricity is the ability of a material to generate a temporary voltage when heated or cooled. When a tourmaline crystal is warmed, one end develops a positive electrical charge while the opposite end develops a negative charge.
This thermal change in electrical polarization was historically noted by the Dutch, who observed that heated tourmaline crystals could attract ash and dust particles. Piezoelectricity is a related phenomenon where an electrical charge is generated in response to mechanical stress or applied pressure. If a tourmaline crystal is compressed along its main axis, it produces an electrical potential.
Both the pyroelectric and piezoelectric effects are a consequence of the mineral’s unique structure, which has a polar axis. The alignment of the internal dipoles within the crystal structure is permanently oriented. Any change in temperature or pressure shifts this alignment, creating a measurable surface charge. This property has led to tourmaline’s use in pressure-sensing devices and other scientific instruments.
Another notable characteristic is strong pleochroism, an optical property where the perceived color of the crystal changes significantly depending on the angle from which it is viewed. This occurs because the crystal absorbs light differently along its different crystallographic axes. For example, a tourmaline crystal may appear dark green when viewed parallel to its long axis, but a much lighter color when viewed perpendicular to it. This optical behavior requires skilled cutting of gemstones to maximize the most desirable color.
Major Varieties and Geological Context
The various common names for tourmaline represent different species or varieties defined by their primary chemical components. Schorl, the most common variety, is black and iron-rich. Elbaite is the lithium-rich variety responsible for most of the gem-quality material, including the red-to-pink variety known as rubellite. Indicolite is the name given to blue tourmaline, while dravite is a magnesium-rich species that is typically brown to yellow-brown. The distinct colors are a direct consequence of the chemical element occupying specific sites within the crystal lattice; for instance, copper causes the intense neon-blue color characteristic of Paraíba tourmaline.
High-quality, gem-forming tourmaline is typically found in two main geological settings. The most important source is granitic pegmatites, which are coarse-grained igneous rocks that form from the final, volatile-rich stages of magma crystallization. These environments concentrate elements like boron and lithium, which are necessary for tourmaline formation, often resulting in large, well-formed crystals.
Tourmaline also forms in metamorphic rocks, such as schists and marbles, where it grows under intense heat and pressure. Magnesium-rich varieties like dravite are more commonly associated with these metamorphic environments. The mineral’s ability to incorporate so many different elements makes it a valuable indicator to geologists, reflecting the specific chemical conditions of the rock formation process.