What is Tourmaline?
Tourmaline is a gemstone that is available in many different colours and falls under the cyclosilicate category of silicate minerals. Chemically, the tourmaline crystal is a boron silicate mineral compound that can have aluminium, iron, magnesium, sodium, lithium, and potassium present in it. The tourmaline stone was first discovered in Sri Lanka, and the Dutch East India Company mined large amounts of the tourmaline crystal and transported it back to their country.
The word tourmaline comes from the Sinhalese word “toramalli” which is a common term for reddish-brown semi-precious gemstones. Tourmaline colour varies greatly, and largely depends on its composing elements. Tourmaline colour is black, deep brown, or bluish-black if it is iron-rich. If it is a magnesium-rich tourmaline stone variety, then it might be brown or yellow. Sometimes, the tourmaline crystal can even be colourless.
Properties of Tourmaline
Depending on the tourmaline mineral, the stone can have many colours, such as blue, green, red, yellow, pink, etc. The tourmaline crystal can also be bi-coloured or multi-coloured, which is quite common. A special type of tourmaline stone, called watermelon tourmaline, has green on the outside and pink on the inside, resembling a watermelon. Some tourmaline varieties can also seem to have different colours when viewed from different directions. This phenomenon is called dichroism. The tourmaline colour can be altered by heat treatment. Heat-treating can lighten dark red stones. They can also be darkened irradiating with gamma rays. Manganese containing pale pink stones can be darkened to a great extent by this technique. However, heat-treated tourmalines are much less than untreated ones.
Tourmaline has the special property of pyroelectricity, which enables the stone to first attract and then repel hot ashes. When rubbed or heated, the tourmaline acquires an electric charge. Due to this special ability, the tourmaline mineral was nicknamed “ Ceylonese Sri Lankan magnet”. Since tourmalines are only slightly brittle, they are excellent for making jewellery.
Sources of Tourmaline
In the 1500s, Portuguese traders; in search of gold; obtained tourmaline from Brazilian indigenous people, and mistook it for emerald and sapphire. They took the stone back to their country and used it to make crown jewels and jewellery pieces for the royalty and the wealthy of the society. Today, Brazil is the largest producer of the tourmaline mineral. The Brazilian state of Minas Gerais has a rich deposit of pegmatite( an igneous rock), in which tourmaline is found. Several African countries also produce large quantities of tourmaline, such as Madagascar, Namibia, Mozambique, Tanzania, Nigeria, and Malawi.
In the United States, southern California is one of the leading producers of tourmaline. Since the 1800s, tourmaline has been the most important gemstone mined in California. Small amounts of tourmaline are also found in Sri Lanka, India, Pakistan, and Afghanistan.
Types of tourmalines
Tourmaline is a gemstone that has found wide usage in jewellery making due to its beautiful and varied colours. It is appreciated by both gem collectors and jewellery lovers for its vibrant appearance and appeal. Tourmalines are mainly of five species: schorl, uvite, dravite, liddicoatite, and elbaite. Of this, schorl or black tourmaline is most easily found.
Elbaite tourmaline, coloured bright green and bright blue due to the presence of copper, was found in Paraiba and Rio Grande do Norte, both states of Brazil in the 1990s. The Paraiba tourmaline is rare and very expensive. Similar variations were later found in Nigeria and Mozambique.
Black Tourmaline
Black tourmaline is also known as Schorl. Unlike other forms of tourmaline, schorl is never transparent or translucent. One of the most common forms of tourmaline, schorl forms in beautiful crystals and has high aesthetic value. It is one of the most lustrous black minerals found on Earth. In the Tourmalinated Quartz, schorl may form as tiny needles.
Schorl is most commonly found in granite pegmatites. It is usually brittle and found as heavily striated prismatic crystals. Its crystals may have a triangular cross-section. Fluor Schorl is a rare form of black tourmaline with a high quantity of fluorine.
The best quality schorls are mined in Shigar Valley and Stak Nala in Pakistan. Afghanistan, Namibia and Madagascar are also schorl producers. In Europe, schorl is produced by Italy, Portugal, the Czech Republic and Sweden.
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Benefits of Black Tourmaline
Schorl is very popular among gem collectors because of its beautiful lustre.
Schorl is used by jewellers as a black gemstone in jewellery.
Schorl-containing tourmalinated quartz is often used as a minor gemstone.
FAQs on Tourmaline
1. What is the fundamental chemical composition of tourmaline?
Tourmaline is not a single mineral but a group of complex boron-silicate minerals. Its basic structure is consistent, but its chemical composition varies significantly, leading to its many types and colours. Key elements that can be present include sodium, calcium, aluminium, iron, magnesium, lithium, and potassium. For example, iron-rich tourmalines are typically black or dark brown, while lithium-rich varieties can be found in a rainbow of colours.
2. What are the main types of tourmaline found in nature?
The main types of tourmaline are classified based on their chemical composition. Some of the most common varieties include:
- Schorl: The most common type, it is iron-rich and typically black in colour.
- Elbaite: A lithium-rich variety known for its vibrant colours (pink, green, blue) and often used as a gemstone.
- Dravite: A magnesium-rich tourmaline, usually brown in colour.
- Paraíba: A rare and highly valuable copper-bearing Elbaite, famous for its intense neon-blue or green hues.
- Rubellite: The red or pink variety of Elbaite.
- Indicolite: The blue variety of Elbaite.
3. How is tourmaline formed from a geographical perspective?
Tourmaline primarily forms in igneous and metamorphic rocks. The most significant source is pegmatites, which are coarse-grained igneous rocks that form during the final stages of magma crystallisation. The hot, water-rich fluids in these environments are rich in the elements needed for tourmaline growth, such as boron and lithium. It can also form in metamorphic rocks like schist or marble when boron from surrounding rocks is introduced during high-pressure and high-temperature conditions.
4. Why does tourmaline exhibit such a wide spectrum of colours?
Tourmaline's remarkable colour diversity is due to its complex and variable chemical structure. The mineral's crystal lattice can easily accommodate different trace elements through a process called isomorphism, or elemental substitution. The presence of these different elements directly influences the stone's colour:
- Iron (Fe): Causes black, brown, and dark blue colours.
- Manganese (Mn): Produces pinks, reds, and sometimes yellows.
- Chromium (Cr) or Vanadium (V): Results in rich green hues.
- Copper (Cu): Creates the vivid neon blues and greens of Paraíba tourmaline.
Changes in the chemical environment during crystal growth can also lead to multiple colours within a single crystal, such as the famous Watermelon Tourmaline.
5. What are the main industrial and scientific uses of tourmaline?
Beyond its use as a gemstone, tourmaline has important scientific and industrial applications due to its unique physical properties. It exhibits both piezoelectricity (generates an electric charge under pressure) and pyroelectricity (generates an electric charge when heated or cooled). These properties make it useful in manufacturing pressure gauges for scientific instruments, high-frequency circuits, and other specialised electronic equipment.
6. How can you differentiate tourmaline from other similar-looking minerals?
Geologists use several key physical properties to identify tourmaline. A primary indicator is its crystal shape; tourmaline crystals are typically elongated prisms with a distinct, rounded triangular cross-section. Other identifying characteristics include:
- Hardness: It has a Mohs hardness of 7 to 7.5, making it quite durable.
- Striations: The crystals often have fine parallel lines, or striations, running along their length.
- Pleochroism: It often shows different colours or depths of colour when viewed from different angles, a property that is very strong in many varieties.
- Fracture: It exhibits a conchoidal to uneven fracture pattern when broken.
7. What makes certain tourmaline varieties, like Paraíba, so much more valuable than others?
The value of a tourmaline variety is determined by a combination of factors, primarily rarity, colour, and clarity. Varieties like Paraíba tourmaline command extremely high prices because they are exceptionally rare and their intense, neon-like colour—caused by traces of copper—is not found in other gemstones. In contrast, Schorl (black tourmaline) is the most abundant and common variety, making it much less valuable. Other factors that increase value include intense, saturated colour, high clarity with few inclusions, and the presence of optical phenomena like chatoyancy (the cat's-eye effect).
