Tourmaline is a diverse crystalline boron silicate mineral. Its complex chemical makeup often includes elements such as aluminum, iron, magnesium, sodium, lithium, or potassium, contributing to its wide array of colors.
Defining Physical Attributes
Tourmaline crystals typically exhibit an elongated, prismatic habit with prominent vertical striations and a distinctive rounded triangular cross-section. Tourmaline ranks between 7 and 7.5 on the Mohs scale of mineral hardness, indicating good scratch resistance. Its surface typically possesses a vitreous, or glassy, luster, though some specimens can appear resinous.
Its extensive color spectrum is a remarkable feature, encompassing nearly every hue, from colorless to black. Tourmaline crystals can display multiple colors within a single specimen, sometimes as distinct color zones or as bi-colored or tri-colored formations. Its transparency varies from fully transparent to opaque, depending on composition and impurities.
Remarkable Electrical and Optical Features
Tourmaline possesses unique electrical properties. One such property is piezoelectricity, meaning it generates an electric charge when subjected to mechanical stress. This allows the mineral to convert mechanical energy into electrical energy. Similarly, tourmaline exhibits pyroelectricity, producing an electric charge when heated or cooled. This thermal sensitivity causes the crystal ends to attract dust particles.
Beyond its electrical responses, tourmaline also displays notable optical phenomena. Many tourmaline crystals show pleochroism, where their color appears different when viewed from various angles. This effect means that light absorption varies depending on the crystal’s orientation, leading to distinct color changes. Some specimens, particularly those with green or blue hues, can exhibit strong pleochroism, appearing darker when viewed along their vertical axis.
The Chemistry Behind Tourmaline’s Colors
Tourmaline is not a single mineral but rather a group of closely related minerals that share a common crystal structure, yet vary significantly in their chemical makeup. This complex and variable composition is directly responsible for the vast array of colors observed in tourmaline. Trace elements incorporated into the mineral’s structure during its formation dictate its specific hue.
For example, iron often contributes to black, dark, blue, or green colors. Manganese is typically responsible for pink, red, and yellow shades, with its concentration influencing the color’s intensity. Green tourmalines often owe their color to the presence of chromium or vanadium. Copper is the coloring agent for neon blue and green hues found in Paraíba tourmaline. Lithium can contribute to a wide range of colors, including various pinks, blues, and greens.
Several well-known varieties of tourmaline are distinguished by their dominant chemical components and resulting colors. Schorl is the most common variety, typically black and rich in iron. Elbaite encompasses many gem-quality tourmalines, characterized by their lithium content and a broad spectrum of colors, including rubellite (pink/red), indicolite (blue), and verdelite (green). Dravite is a brown tourmaline, typically rich in magnesium. Watermelon tourmaline is a distinctive bi-color variety that often features a pink core surrounded by a green outer layer, resembling the fruit.
Utilizing Tourmaline’s Unique Traits
Tourmaline’s varied properties lead to several practical applications. Its beauty, wide color range, and reasonable durability make it a popular choice for jewelry. Jewelers often facet tourmaline into various cuts or polish it as cabochons. Its hardness, ranging from 7 to 7.5 on the Mohs scale, allows it to withstand daily wear in rings, necklaces, and earrings.
Beyond its ornamental uses, tourmaline’s electrical properties find application in scientific and technological fields. Its piezoelectric nature makes it valuable in pressure gauges and other instruments designed to detect and measure pressure variations. Historically, this property has been used in devices like depth-sounding apparatus. The pyroelectric property has also led to its use in some heating elements or air purifiers.