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Chemical Formula | "Abbreviated formula" (covers only
Elbaite, Schorl, and Dravite): (Na,Ca) (Mg,Li,Al,Fe2+)3 Al6 B3 Si6 (OH)4 "Expanded formula" (covers Elbaite, Schorl, Dravite, Liddicoatite, Uvite, Chromdravite, and Buergerite): (Na,Ca) (Mg,Li,Al,Fe2+,Fe3+)3 (Al,Mg,Cr)6 B3 Si6 (OH,O,F)4 The formula for the Tourmaline group is one of the most complex in the entire mineral kingdom. See The chemical formula of Tourmaline for a clarification of this subject. |
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Composition | See The chemical formula of Tourmaline | ||
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Color | Black, brown, green, red, blue, yellow, pink, purple. Rarely orange, white, or colorless. Crystals frequently contain two or more colors. Some specimens are pleochroic. | ||
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Streak | White | ||
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Hardness | 7 - 7½ | ||
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Crystal Forms and Aggregates |
(Hexagonal) Usually occurs as heavily striated, elongated prismatic crystals, and less commonly as short, stubby, prismatic crystals. All Tourmaline crystals have a rounded, triangular cross-section. Seldom occurs in tabular crystals. Also occurs columnar, radiating, stalactitic, in dense groups of tiny, elongated needles, and in black, compact masses. | ||
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Transparency | Transparent to opaque | ||
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Specific Gravity | 3.0 - 3.3 | ||
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Luster | Vitreous. Some black and brown specimens are dull. | ||
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Cleavage | 3,2 | ||
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Fracture | Conchoidal to uneven | ||
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Tenacity | Brittle | ||
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Other ID Marks | 1) Strongly pyroelectric 2) May fluoresce yellow in shortwave ultraviolet light 3) Piezoelectric |
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Varieties | There are some members of the Tourmaline
group that are scientifically classified as different
minerals, since they contain different elements in their structure. However, they are all isomorphous and are very similar in physical
characteristics. These are: Elbaite Schorl Dravite Liddicoatite Uvite Chromdravite Buergerite (For the chemical formula of these varieties, see The chemical formula of Tourmaline) There are a few very rare members of the Tourmaline group which are not discussed in this guide. Some varieties are named for their color, rather than for their chemical composition. Some of the common names used both in the gemstone trade and in the mineral market are: Rubellite - pink to red variety Indicolite - blue variety Schorl - black variety Dravite - brown variety Achroite - colorless variety Watermelon Tourmaline - variety that is green on the outside, and red on the inside (or vice versa) Schorl and Dravite are separate minerals, but are commonly categorized just by their color. Elbaite is an independent member of the Tourmaline group, and its color varies greatly. In the gem trade, however, Elbaite refers to green Tourmaline, and Tourmaline by itself also refers to green Tourmaline. Variety names less commonly used: Verdelite - green variety Siberite - purple variety Paraiba Tourmaline - neon-blue variety (found in Paraiba, Brazil) Moor's Head Tourmaline - Lightly tinted, transparent Tourmaline with a black top (found on the island of Elba, Italy) All the above varieties, except for Schorl and Dravite, are varieties of Elbaite Tourmaline. |
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In Group | Silicates ; Cyclosilicates | ||
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All About | Tourmaline is a very popular gemstone and
collectors mineral. It exists in all colors, and the
multicolored stones are unsurpassed in splendor. Tourmaline is both pyroelectric and piezoelectric. If a specimen is put under a pressure or temperature change, it will generate an electrical charge. (Dust particles will become attached to crystal ends in such cases). Tourmaline was used in the 1700's and 1800's to clean out ash and dust from smoking pipes. 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. 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. Fine Tourmaline crystals are very valuable to the collector. Instead of being faceted, many specimens are preserved for their beauty. Thick, elongated crystals are sometimes sliced into sections and sold as "Tourmaline cross sections". |
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Uses | Tourmaline is a very popular gem. Almost all
gem tourmaline is of the Elbaite variety. Elbaite also
contains the appealing multicolored crystals. Fine
crystals are very expensive, as they make beautiful
specimens. They are one of the most prized minerals to a
collector. The varieties Schorl and Dravite are usually opaque and have little gem value. Both are common and not particularly interesting. In the past, mourning jewelry was carved out of black Schorl. The other forms of Tourmaline (Liddicoatite, Uvite, Chromdravite, and Buergerite), are very rare and are only appreciated by serious mineral collectors. Tourmaline often occurs in long thin needles densely included inside Quartz. Such material is known as Tourmalinated Quartz. Tourmalinated quartz is carved into valuable ornamental objects and figures. The piezoelectric habit in Tourmaline makes it an important component of high pressure gauges. See the gemstone section on tourmaline |
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Striking Features | Color and crystal forms, hardness, and deep vertical striations | ||
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Popularity (1-4) | 1 | ||
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Prevalence (1-3) | 1 | ||
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Demand (1-3) | 1 | ||
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Distinguishing Similar Minerals |
Beryl - striations are much finer and are horizontal Apatite - crystals lack striations, softer (5) Hornblende - lacks triangular cross-section. Also slightly softer. Amphibole and Pyroxene minerals - have distinct cleavage |
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Commonly Occurs With |
Quartz, Microcline, Lepidolite, Beryl, Spodumene (Kunzite), Biotite, Siderite, Calcite | ||
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Noteworthy Localities |
Elbaite is named for the Island of Elba,
Italy, where many fine specimens were found, including
the famous "Moor's Head Tourmaline". The
largest producer of gem quality Tourmaline is Minas
Gerais, Brazil. Many magnificent museum specimens have
come from there. Significant deposits containing huge gem
crystals occur in Madagascar. Other notable gem deposits
exist in Sri Lanka, Russia, Myanmar (Burma), Afghanistan,
Pakistan, Nepal, and Namibia. Enormous Dravite crystals
were found in Yinniethara, Western Australia. Fine color
zoned crystals have come from Mexico at Alamos, Baja
California. Stalactitic Schorl with a velvety luster was
recently found in Santa Cruz, Sonora, Mexico. In the U.S., beautiful multicolored crystals have come from the famous Tourmaline mines in the Pala area, San Diego Co., California. Also famous in California is the Little Three mine, Ramona, San Diego Co. Newry (Mount Mica area), Oxford Co., Maine has also produced excellent quality specimens. Fine Dravite and Schorl specimens have come from St. Lawrence Co., New York, in DeKalb, Pierrepont, and Gouverner. Uvite has been found in Gouverner, St. Lawrence Co., New York, and in Ogdensburg and Hamburg, Sussex Co., New Jersey. The best Liddicoatite crystals come from Madagascar (at Alakamisy - Itenina [Fianarantsoa]). The rare variety Buergerite comes from San Luis Potosi, Mexico. |
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Picture Icon Links |
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Picture Links | 1. Green Elbaite on white Calcite |
