 | | | Tourmaline is not a single mineral, but a group of several closely related minerals. The three most well-known members are Elbaite, Schorl, and Dravite. Other lesser known members include Uvite, Liddicoatite and Buergerite. There are yet more members of the Tourmaline group, but these are extremely rare and not discussed in this guide. Tourmaline is extremely popular among collectors and is a well-known gemstone. It is the most multicolored mineral type known, occurring in virtually every color of the spectrum. Individual stones are often multicolored and are unsurpassed in their beauty. The color of some Tourmaline can be enhanced through heat treatment. Some greenish stones can be made deep green, some brownish-red stones can be made red, and some light pink stones can be made colorless through heating. Tourmaline has many interesting optical properties. Many green and blue specimens are strongly pleochroic. When viewed through their vertical axis, such specimens appear darker in color than when seen through their horizontal axis. In other Tourmalines, the color may actually be different when viewed at different angles because of the pleochroism. Certain Tourmalines exhibit a cat's eye effect when polished into cabochons. Tourmaline is both pyroelectric and piezoelectric. If a specimen is put under a pressure or temperature change, it will generate an electrical charge. When this happens, dust particles become attached to the crystal ends. For additional information, see the gemstone section on Tourmaline. | |
| Chemical Formula | The simple chemical formula, which covers the main forms of Tourmaline (Elbaite, Schorl, and Dravite), is as follows:
(Na,Ca)(Mg,Li,Al,Fe2+)3Al6(BO3)3Si6O18(OH)4The expanded formula, which additionally covers Uvite, Liddicoatite, and Buergerite, is as follows:
(Na,Ca)(Mg,Li,Al,Fe2+,Fe3+)3(Al,Mg)6(BO3)3Si6O18(OH,O,F)4 The formula for the Tourmaline group is very complex. See The chemical formula of Tourmaline for more details. | | Composition | See The chemical formula of Tourmaline. | | Color | Tourmaline is extremely varied in color. Colors include black, brown, green, red, pink, blue, and gray. White, colorless, yellow, orange, and purple colors are less common. Crystals are frequently multicolored, containing two or more distinct colors. Some specimens are pleochroic. | | Streak | White | | Hardness | 7-7.5 | | Crystal System | Hexagonal | 3D Crystal Atlas
(Click for animated model) | | | Crystal Forms and Aggregates | Usually as elongated prismatic crystals that are heavily striated. Also as short, stubby, prismatic crystals. Most Tourmaline crystals have a rounded, triangular cross-section. Seldom in tabular crystals. Aggregates include columnar, radiating, botryoidal, stalactitic, in dense groups of tiny, elongated needles, and in compact masses. | | Transparency | Transparent to opaque | | Specific Gravity | 2.9-3.3 | | Luster | Vitreous. Some black and brown specimens may be dull. | | Cleavage | 3,2 | | Fracture | Conchoidal to uneven | | Tenacity | Brittle | | Other ID Marks | 1) Strongly pyroelectric.
2) Piezoelectric.
3) A few forms of Tourmaline fluoresce yellow in shortwave ultraviolet light.
| | In Group | Silicates; Cyclosilicates; Tourmaline Group | | Striking Features | Color, crystal form, hardness, and deep vertical striations. | | Environment | Elbaite, Schorl, and Liddicoatite are almost exclusively from granite pegmatites, while Dravite and Uvite or mostly from metamorphic environments such as marbles. Buergerite is from igneous rhyolite deposits.
| | Rock Type | Igneous, Metamorphic | | Popularity (1-4) | 1 | | Prevalence (1-3) | 2 | | Demand (1-3) | 1 | |
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| | | The variety list below shows the main Tourmaline group members, as well as the popular Elbaite variety forms. Please see individual member page for additional variety names.
-Colorless variety of Elbaite Tourmaline.
-Rare individual member mineral of the Tourmaline group, occurring almost exclusively at Mexquitic, San Luis Potosí, Mexico.
-Individual member mineral of the Tourmaline group. It is usually brown in color, and the term may be corrupted to include all forms of brown Tourmaline.
-The most well-known individual member mineral of the Tourmaline group. Elbaite is the most transparent and colorful form of Tourmaline. The term Elbaite may be corrupted in the gemstone industry to refer specifically to green Tourmaline.
-Blue variety of Elbaite Tourmaline.
-Uncommon member mineral of the Tourmaline group found primarily in Madagascar. It is the calcium analogue of Elbaite, containing calcium in its chemical formula instead of sodium.
-Pink to red variety of Elbaite Tourmaline.
-Individual member mineral of the Tourmaline group. It is black in color, and the term may be corrupted to include any very dark Tourmaline forms.
-Uncommon member mineral of the Tourmaline group, usually found in metamorphic environments such as marbles.
-Variety of Elbaite Tourmaline that is green on the outside and red on the inside.
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| | | Tourmaline is a very popular gemstone. Its popularity has greatly increased since the 1990's, and this has also been reflected in the astronomical prices demanded from the finest Tourmalines. Tourmaline is used as a jewelry gemstone and is cut into all forms and styles. All colors of Tourmaline, especially multicolored gems, are used in jewelry. Tourmaline can form in extremely aesthetic slender crystals that are highly valued by collectors. It is one of the most prized minerals, and fine crystals can be among the most beautiful examples in the mineral kingdom. Instead of being faceted, many fine Tourmaline crystals are preserved for their beauty. Thick, elongated crystals are sometimes sliced into sections and sold as "Tourmaline cross sections". The piezoelectric nature of Tourmaline makes it useful as a component of high pressure gauges. |
| | See the individual Tourmaline mineral pages for detailed locality information for each form of Tourmaline.
Elbaite|Schorl |Dravite |Uvite|Liddicoatite|Buergerite |
| COMMON MINERAL ASSOCIATIONS | | | | Quartz, Microcline, Albite, Lepidolite, Beryl, Spodumene, Biotite, Cookeite, Calcite |
| DISTINGUISHING SIMILAR MINERALS | | | Beryl - Striations are much finer and are horizontal, and usually more hexagonal in crystal form.
Apatite - Crystals lack striations , softer (5).
Epidote - Softer, different crystal habits. |
| | | The pictures below show just one example of each individually named type of Tourmaline. For more images, see the specific pages on Elbaite, Schorl, Dravite, Uvite, Liddicoatite, and Buergerite. |
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