GO 340 Gemstones & Gemology
Emporia State University

academic.emporia.edu/abersusa/go340/tourmali.htm

Tourmaline


Image taken from
the former Gallery of Gemstones.
It is rare to tourmaline crystals of only one color. However, the tourmaline group of minerals has a number of varieties, with the most common being indicolite (blue), rubellite (pink to red), schorl (black, and commonly used in mourning jewelry), and verdelite (green in all shades). Common properties include a hardness of 7-7.5, specific gravity of 2.82-3.32, and uneven to small conchoidal fracture, with brittle tenacity.

By heat treating tourmaline within a range from 842 to 1,202 degrees F, a color change can be produced in some tourmaline. Some green tourmalines can be darkened and some are lightened by this process. Gamma-radiation can be used to change the color of tourmaline, but often this color change will fade over time (Schumann, 2011, p. 128).


Pink Tourmaline.
Image taken from
the former Gallery of Gemstones.
Bi-colored tourmaline crystals are common and the stones with a red core and green skin are called watermelon tourmaline. The crystal system is trigonal-hexagonal and may be obvious in a natural crystal but usually not after fashioning the gem. It is reflected in the common prismatic crystal forms with triangular or hexagonal cross sections.

Bi-colored Tourmaline.
Image taken from
Gem Hut.
Tourmaline is doubly refractive with refractive indices of 1.614 and 1.666.
Although the fluorescence is weak to none, pleochroism is detected in
the different varieties. Tourmaline that is nearly opaque and fibrous can
be fashioned to display chatoyancy or the cat's eye effect. This may be found
in both pink and green varieties.

Gem tourmaline deposits are found in pegmatites
and alluvial accumulation located downgradient from
the igneous host rock. Currently, the main supplier of
tourmaline is Brazil. Other countries that produce this
gem mineral include: Afghanistan, Australia, Burma
(Myanmar), India, Madagascar, Malawi, Mozambique,
Namibia, Nepal, Nigeria, Pakistan, Russia, Zambia, Zimbabwe, Sri Lanka, Tanzania,
United States, Zaire, Italy, and Switzerland (Schuman, 2011, p. 128). Tourmaline is an October birthstone.


Pink Cat's Eye Tourmaline.
Image taken from
Gem Hut.

Tourmaline is pyro- and piezo-electric. This means that when it is rubbed, heated/cooled and pressure applied, the crystal will become electrically charged. It was known in times past as the ash puller, because this remarkable property allowed one to pull ash from meerschaum tobacco pipes! However, this attraction for particles and dust means it must be cleaned more often than other gems. Also, this piezoelectric property means tourmaline is valued in industry for pressure gauges and the like; synthetic tourmaline is used for industrial, not gem, purposes.

The image to the right is Indicolite Tourmaline that has not been treated. The image was taken from Gem Hut.

References

Additional Sites

Return to the Syllabus or choose another gemstone below.

Alexandrite Amber Amethyst Chalcedony Diamond Emerald
Garnet Jade Malachite Opal Pearl Peridot
Ruby Sapphire Tanzanite Topaz Tourmaline Turquoise

This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: esu.abersusie@gmail.com Thanks for visiting! Webpage created: November 15, 2000; last update: November 19, 2012.

Copyright 1999-2012 Susan Ward Aber. All rights reserved.