Introduction and General Overview
Tourmalines are arguably one of the most popular gemstones ever discovered. Originally native to Sri Lanka, significant finds have recently been made across Europe, Africa and South America depicting the inherent multiplicity of this mineral species. With a wide array of dazzling color variations that span from black, red, and pink, to even peach, tourmalines have become an international favorite for many jewel lovers. In particular, the blue-green copper-bearing Paraiba-type is one of the most noticeable and highest priced gems from the tourmaline family with quality specimens fetching as much as $300 and $600 per carat (“Paraíba Tourmaline Value, Price, and Jewelry Information,” n.d.). Demand for tourmaline was notably significant during the Second World War (1939-45) since it was an integral component in the development of war-time equipment such as submarine pressure sensitive gauges. Later on, tourmalines encountered a receptive environment that bolstered its popularity and permeation across the globe.
Color Producing Mechanism
Tourmalines exhibit an extensive range of colors. Typically, specimens may be black, red, pink, yellow, blue, green or even multicolored. In essence, this kaleidoscope of colors is a reflection of the chemical interactions that have taken place in the gemstone at specific structural levels. Commonly referred to as the inter-valence charge transfer interactions (IVCT) and the rare crystal field transitions (CFT), these primary causative agents are the main color producing mechanisms. Transition elements such as ferrous ions, manganese (II) ions, titanium (4+) ions, cupric ion, and vanadium 3+ ions are some of the most notable color-causing agents. Commonly referred to as chromophores, they are located in the Y and Z segment of the octahedral sites and responsible for the color intensity evident in the gem. In addition to this, natural irradiation is also a chief factor in the enhancement of these naturally occurring colors. Decaying isotopes (markedly 232Th, 40k, and 238U) may be found close to sites rich in tourmaline inadvertently influence the gem’s color.
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During other instances, tourmaline may present chatoyancy. In this particular state, the gem scatters parallel bundles of light rays hitting it and ultimately aligns it with its c axis. It is, however, vital to acknowledge that the intensity of this spectacle is dependent upon the dimension and density of the tubes located in each crystal. Nevertheless, changing conditions have also been extensively explored as crucial factors for consideration when examining the tourmaline color producing mechanism. According to Klein & Philpotts (2016), changes that occur during the crystals development may result in any of the different colors observed in various tourmaline species (183). During its nascent growth period, a color may overgrow and result in bicolor crystals with a series of zones through its cross-section. Through such an intricate mechanism, collectors have been successful at amassing some of the most desirable specimens that also happen to be novelty samples.
Physical Variations and Production
The variation observed in the physical differences of separate tourmaline species is as a result of the gem’s elaborate chemical makeup. In all, there are 30 unique tourmaline species, with each being a discrete derivative of the XY3Z6 (T6O18) (BO3)3V3W chemical formula (Dietrich, 2012). Each has the same chemical structure, even though their internal elements may vary from one species to another. Colors transition from the more common deep-green right through to the rare deep-red as the light wavelength increases. Aptly dubbed the “Usambara effect,” the physical variations are attributes of various spectral positions in specific ratios during transmission.
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Even so, all versions of the gemstone occur in metamorphic and igneous rocks. Achroite is the rarest of the Elaite tourmaline gem variety and colorless. Another scarce individual is the bronze-like buergerite tourmaline that was first discovered in 1966 by exploration geologists in San Luis Potosí, Mexico. Columnar aggregates result in the production of three-sided prisms that develop growth layers over time. Dravite tourmaline is rich in sodium magnesium, hence its brown color, though color variations also exist in this species. They may appear as Indicolite (blue tourmaline), Siberite (scarlet-violet tourmaline), Rubellite (red tourmaline), Watermelon tourmaline (iridescent pink) and Paraiba (blue-green tourmaline). Physical variations also extend to the Elaite tourmaline group well-known for its value. Traces of impurities found in the crystal tint the gemstone, making it allochromatic, although it can also appear pleochroic. Multi-color zones are, thus, common here giving the gem a rainbow allure. Final products often acicular inclusions that are microscopically resulting in the infamous cat eye effect. Schorl tourmaline represents the only black variation in the group while color zoned liddicoatite are produced against a backdrop of parallel pyramid faces.
Crystal Structure and Molecular Composition
Tourmaline crystal structure constituting six-fold tetrahedra rings atop an octahedra plane consisting of three Y polyhedral and six Z
Uses of Tourmaline
Tourmaline’s popularity has soared over the past two decades. It is now commonplace for buyers to part with a substantial amount of money to obtain these rare natural specimens. Even so, buyers still purchase this rare gem for a range of reasons.
Firstly, tourmaline is commonly cut into various styles and used as jewelry gemstones. Its wealth of colors has made it a favorite for many collectors since buyers have a wide array of colors from which to choose from. Its aesthetic value has made it the most prized gem in the mineral kingdom and still reigns supreme owing to its magnificent beauty. Tourmaline gems are never identical, a factor that contributes to its general appeal and demand among collectors. Tourmaline is fashioned into bracelets, necklaces, rings, and pendants that are worn by its admirers across the globe.
Secondly, tourmaline acts as an electrical conductor when heated. This is because it is piezoelectric, enabling it to hold electrical charges once heated and consequently cooled. Experts specializing in the minerals uses have harnessed its unique characteristics and now use it in blow dryers and hair straighteners. The negative ions produced once it is heated reduce fizz which then shields the hair from heat-induced damage. Adding the mineral to blow dryers results in the discharge of more ions while remaining light and manageable.
Thirdly, tourmaline is a polarizing device. Cutting the mineral along two opposite axes allows it to act as a device capable of blocking light and the reason why manufacturers use it to build tongs. Moreover, this particular attribute can also be used in concert with its ability to conduct electricity resulting in a device for measuring and monitoring pressure changes. Gauges have benefitted the most from the incorporation of this particular gemstone. Temporary spikes in pressure can now be easily detected, thus improving personnel safety.
Conclusion
Tourmalines represent an exclusive class of semi-precious minerals famed for their striking glamour and beauty. They include elements such as lithium, magnesium, aluminum, sodium, and iron which influences its color. As a result, a broad spectrum of colors ranging from black, red, pink, yellow, blue to green. These colors are usually as a result of the chemical interactions occurring in the gemstone at a structural level. The inter-valence charge transfer interactions (IVCT) and crystal field transitions (CFT) are the two predominant color producing mechanisms. The tourmaline group includes gemstones with a wide range of variations due to its elaborate chemical makeup. These dissimilarities have allowed experts to fashion the gem into various pieces of jewelry, use it as an electrical conductor and also as a polarizing device.
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