tourmaline

Tourmaline, a mesmerising gemstone, derives its name from the Sinhalese word "turamali," meaning "stone with mixed colours." Comprising a complex borosilicate mineral group, tourmaline showcases a remarkable array of colours, making it a favourite among gem enthusiasts and collectors.

Geologically, tourmaline forms in a variety of environments, including granite and pegmatite rocks. Notable sources include Brazil, Madagascar, and Afghanistan, each yielding unique colour varieties.

What sets tourmaline apart is its exceptional range of colours, often displaying bi-coloured or tri-coloured crystals. Tourmalines are renowned for their intricate internal growth structures, which can create striking cat's-eye and watermelon-like patterns when cut and polished.

One interesting fact about tourmaline is its unique property of pyroelectricity, meaning it can generate an electrical charge when heated or subjected to pressure changes. This property has led to its use in scientific instruments and mineralogical studies. With its vibrant hues and fascinating properties, tourmaline continues to captivate gem connoisseurs worldwide.

Click on the terms in the table below to discover their meaning

Click on the terms in the table below to discover their meaning

Name Tourmaline
Category Cyclosilicate
Chemical Formula (Ca, K, Na, Pb, ▢)(Al, Fe2+, Fe3+, Li, Mg2+, Mn2+, Ti)3(Al, Cr3+, Fe3+, V3+)6(BO3)3(Si, Al, B3+)6O18(O, OH)3(F, O, OH)
Note: ▢ represents an atomic vacancy
IMA Symbol Tur
Crystal System Trigonal
Crystal Habit Prismatic to acicular, with prominent trigonal prism and pyramid, commonly hemimorphic, striated; also radial, fibrous, and massive
Twinning Rare
Cleavage Poor
Fracture Conchoidal to uneven
Tenacity Brittle
Hardness
(Moh's Scale)		 7
Specific Gravity 2.90 - 3.10
Diaphaneity Transparent to translucent
Colour Colourless, white, yellow, orange, red, pink, blue, green, brown, black; zoning common parallel to trigonal outline
Streak White or light brown
Lustre Vitreous to resinous
Optical Class Uniaxial (-)
Refractive index nω = 1.634 - 1.661 and nϵ = 1.612 - 1.632 (Dravite)
nω = 1.633 - 1.651 and nϵ = 1.615 - 1.630 (Elbaite)
nω = 1.660 - 1.671 and nϵ = 1.635 - 1.650 (Schorl)
Birefringence δ = 0.014 - 0.040
Dispersion Rd = 0.017
Pleochroism O = pale yellow; E = colourless, yellowish, greenish, brownish (Dravite)
O = pink, pale green, pale to deep blue; E = colourless, yellow, olive-green, purplish (Elbaite)
O = yellow-brown; E = pale yellow, pale brown (Schorl)
Clarity Type II or Type III
Notable Varieties Rubellite, Indicolite, Verdelite, Paraíba Tourmaline, Watermelon Tourmaline, Schorl, Chrome Tourmaline, Cat's Eye Tourmaline (displays chatoyancy), Achroite

References

Hudson Institute of Mineralogy. (2023). Tourmaline. Retrieved from Mindat.org: https://www.mindat.org/min-4003.html

Mineralogical Society of America. (2001). Dravite. In J. W. Anthony, R. A. Bideaux, K. W. Bladh, & M. C. Nichols (Eds.), Handbook of Mineralogy. Chantilly, VA 20151-1110, USA: Mineralogical Society of America. Retrieved from https://www.handbookofmineralogy.org/pdfs/dravite.pdf

Mineralogical Society of America. (2001). Elbaite. In J. W. Anthony, R. A. Bideaux, K. W. Bladh, & M. C. Nichols (Eds.), Handbook of Mineralogy. Chantilly, VA 20151-1110, USA: Mineralogical Society of America. Retrieved from https://www.handbookofmineralogy.org/pdfs/elbaite.pdf

Mineralogical Society of America. (2001). Schorl. In J. W. Anthony, R. A. Bideaux, K. W. Bladh, & M. C. Nichols (Eds.), Handbook of Mineralogy. Chantilly, VA 20151-1110, USA: Mineralogical Society of America. Retrieved from https://www.handbookofmineralogy.org/pdfs/schorl.pdf

About the Author

Daniel Gillespie is student of Web Development and the developer of Gemology 101. With a straightforward and informative approach, Daniel invites readers to explore the world of gemology, offering a valuable resource for both enthusiasts and those seeking knowledge about precious stones.