Monday, April 30, 2012

Tourmaline: a gemstone name with Singhalese roots

The gemstone name tourmaline is though to derive form a Singhalese (Sinhalese, Cingalese) word or a variant thereof: turmali, tourmali, turamali, thuramali and thoramali are some of its spellings that can be found in the literature, buyer's guides and on websites [1-5]. The name means “mixed parcel,” referring to the large variety of color combinations in which its gemstone specimens occur [3,4]. Originally, the term tourmaline was applied to zircon and other gems by jewelers of Ceylon (now Sri Lanka) [5].

Localities for tourmaline specimens include places in Asia, Europe, Africa, North and South America and even Antarctica (map in [2]). Dutch traders, shipping tourmaline stones from Sri Lanka to Europe, called them Aschentrecker (“ash puller”) due to their pyroelectric properties [1]. Tourmaline is also thought to be identical with lyngurium, described by the ancient Greek scholar Theophrastus of Eresus  (c. 371-285 BC) in his work on stones, De lapidibus,—mistaken as solidified lynx urine.

Classified as a semi-precious stone, tourmaline actually comes in different species and varieties. The Dictionary of Geology and Mineralogy [6] defines tourmaline as “any of a group of cyclosilicate minerals with a complex chemical composition, vitreous to resinous luster, and variable color, crystallizes in the ditrigonal-pyramidal class of the hexagonal system, has piezoelectric properties, and is used as a gemstone.” The dictionary gives the general formala,


which has to be modified to a more specific one when particular species are considered. To better understand the crystal structure of tourmalines, one may, for example, consult the CrystalMaker®-generated drawings by  Darrell Hennry and Barbara Dutrow (page 20 in [1]), depicting structural features of one of the most common tourmaline species, schorl (NaFe2+3,Al6Si6O18(BO3)3(OH)3OH). Their figures demonstrate ion and polyhedra arrangement and, in particular, the cyclosilicate-coordinated structure based on rings of six SiO4 tetraeder (the Si6O18 unit in the formula). Further, these authors explain the composition and color zoning structure of tourmalines and their significance as forensic objects in earth science and natural history. 

Keywords: geology, mineralogy, gemstones, lapidary, crystal morphology, linguistics, terminology.

References and more to explore
[1] Darrell J. Henry and Barbara L. Dutrow: The Tourmaline Diaries. Natural History March 2012, 120 (3), 16-27.
[2] Tourmaline [].
[3] Richard W. Hughes: Tourmaline Buying Guide [].
[4] Tourmaline's name... [].
[5] Richard Scott Mitchell: Mineral Names. What do they mean? Van Nostrand Reinhold Company, New York, 1979; page 192.
[6] Dictionary of Geology & Mineralogy. Second Edition. McGraw Hill, New York, 2003.

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