Tourmaline is a group of closely related borosilicate minerals that occurs in nearly every color, often with two or more colors in one crystal. Gem tourmaline is mostly elbaite, the lithium-rich member of the group, and it ranks 7 to 7.5 on the Mohs scale. Its strong pleochroism, long prismatic crystals and electrical properties make it one of the more interesting gems to study as well as to wear.
The tourmaline group
Tourmaline is not a single mineral. It is a family of more than 30 species with the same crystal structure but different chemistry. The structure has room for many elements, including sodium, calcium, lithium, magnesium, iron, aluminum, manganese, chromium, vanadium and copper. Small changes in chemistry shift both the species name and the color.
The members that matter most in gems and jewelry:
- Elbaite is the lithium-rich species and the source of most transparent gem tourmaline in pink, red, green, blue and multicolor.
- Liddicoatite is a calcium-rich species close to elbaite. It is known for crystals from Madagascar that show triangular color zoning when sliced across the length. Named for Richard Liddicoat of GIA, it is hard to separate from elbaite without chemical analysis.
- Dravite is the magnesium-rich species, usually brown to yellow or dark green.
- Uvite is a calcium and magnesium species, often green to brown.
- Schorl is the iron-rich species, almost always black and opaque. It is the most common tourmaline in nature. See schorl.
In 2011, the International Mineralogical Association adopted a revised classification of the group, and some names have changed in technical literature. The trade still uses the variety names below.
Trade names for tourmaline colors
- Rubellite is pink to red tourmaline that keeps its color in both daylight and incandescent light without turning brownish. See rubellite tourmaline.
- Indicolite is blue tourmaline, usually colored by iron. Fine, saturated blue indicolite is uncommon.
- Verdelite is an older name for green tourmaline, the most common gem color.
- Chrome tourmaline is a rich green colored by chromium and vanadium, mostly dravite or uvite from Tanzania and Kenya.
- Watermelon tourmaline has a pink center and a green rim, best seen when crystals are cut in slices across the length.
- Bicolor and parti-colored stones show two or more colors along the length of the crystal.
- Paraiba tourmaline is copper-bearing elbaite with a neon blue to green color. See Paraiba tourmaline.
- Achroite is colorless tourmaline, which is rare.
Why tourmaline has so many colors
The causes of tourmaline color are well understood. Iron gives blue, green and black. Manganese gives pink and red. Copper produces the bright blues and greens of Paraiba material. Chromium and vanadium give the deep chrome greens. Titanium together with iron can add yellow and brown tones.
Color zoning is common because the chemistry of the fluids from which tourmaline grows changes as crystals form. A crystal can start pink and finish green, which creates watermelon and bicolor stones. The page on color in gemstones explains these causes.
Pleochroism and cutting
Tourmaline is strongly dichroic. Most crystals look darker and more saturated when viewed down the length (along the optic axis) and lighter across it. In some green and blue crystals, the color down the length is so dark that the stone looks almost black. Cutters often place the table parallel to the length of the crystal to avoid this, which is one reason tourmaline is frequently cut as long rectangles and ovals. Pale crystals may be cut with the table across the length to deepen the color. The page on pleochroism in gemstones shows how this works.
Tourmaline is doubly refractive, with birefringence of about 0.018-0.020 in most elbaite, large enough to show faint doubling of back facets under a loupe in larger stones.
Pyroelectricity and piezoelectricity
Tourmaline develops an electric charge when heated or cooled (pyroelectricity) and when squeezed (piezoelectricity). Dutch traders in the 18th century noticed that warmed tourmaline crystals attracted ash, and they called the stones "ash pullers." The same property means tourmaline displayed in a warm shop window can attract dust. During the 20th century, tourmaline pressure gauges were used to measure blast pressures, before synthetic materials took over.
Where tourmaline forms and is found
Gem elbaite forms mainly in granite pegmatites, coarse-grained igneous bodies rich in lithium, boron and other rare elements. Pockets within pegmatites can contain large, clean crystals alongside quartz, feldspar, mica and topaz. Dravite and uvite form in metamorphic rocks and marbles. The page on igneous rocks and gemstones explains pegmatites.
Major sources
- Brazil has been the largest producer of gem tourmaline for a long time, especially from pegmatites in Minas Gerais. See gems of Minas Gerais.
- Afghanistan's Nuristan and Laghman provinces produce fine pink, green and blue elbaite from pegmatites. See gems of Afghanistan.
- Nigeria, Mozambique and Madagascar produce large amounts of colored tourmaline, including rubellite and copper-bearing material.
- The United States is a historic source. Pegmatites in San Diego County, California, produced pink tourmaline exported to China in the late 19th and early 20th centuries, where the Empress Dowager Cixi favored it. Maine, where tourmaline was found at Mount Mica in 1820, was an early American gem source.
- Tanzania and Kenya produce chrome tourmaline, and Sri Lanka yields tourmaline in its gem gravels.
Treatments
Heating can lighten overly dark green or blue tourmaline, and moderate heat removes purple tones from some copper-bearing stones. Irradiation can intensify pink and red colors. Both treatments are generally stable and often hard to detect. Fracture filling with oil or resin is used for included stones, especially rubellite. Disclosure norms apply as with other gems; see gemstone treatments.
Durability and care
Tourmaline is hard enough for most jewelry at 7 to 7.5 on the Mohs hardness scale. It has no easy cleavage, but many stones, especially rubellite, contain fractures and liquid inclusions, so toughness is only fair. Clean tourmaline with warm soapy water and a soft brush. Avoid ultrasonic and steam cleaners for stones with fractures or fillings, and avoid sudden temperature changes.
Tourmaline as a birthstone
Tourmaline is one of the October birthstones, alongside opal. The name comes from the Sinhalese word turmali, which was used in Sri Lanka for mixed parcels of gem gravel stones, many of them zircon and other gems.
Frequently asked questions
What is the rarest tourmaline color?
Saturated blue and the neon blue of copper-bearing Paraiba material are among the rarest, along with colorless achroite. Fine red rubellite with no brown tones is also uncommon.
Is tourmaline good for an engagement ring?
It is hard enough for occasional wear and can be worn daily with care, but it scratches more easily than sapphire or diamond. Protective settings and removal during hard work help.
What is watermelon tourmaline?
It is a crystal with a pink center and green outer rim, formed when fluid chemistry changed as the crystal grew. Slices cut across the crystal show the pattern clearly.
Why does tourmaline look darker from one side?
Tourmaline is strongly pleochroic. Viewed along the length of the crystal, it absorbs more light and looks darker, so cutters orient the stone to show the best color through the table.