Tourmaline - October Birthstone

Tourmaline is one of the most commercially important gemstones in the world. 

Somewhere in Brazil in the 1500s, a Spanish conquistador washed the dirt from a green tourmaline crystal and confused the vibrant gem with emerald. His confusion lived on until scientists recognized tourmaline as a distinct mineral species in the 1800s. The gem’s name comes from the Sinhalese word for mixed gems: toromalli. It’s a term Dutch merchants applied to the multicolored, water-worn pebbles that miners found in the gem gravels of Ceylon (now Sri Lanka).

Tourmalines are a group of closely related mineral species that share the same crystal structure but have slightly different chemical and physical properties. Gemologists use a tourmaline’s properties and chemical composition to define its species. The major tourmaline species are elbaite, liddicoatite, dravite, uvite, and schorl. 

Tourmaline’s colors have many different causes. It’s generally agreed that traces of iron, and possibly titanium, induce green and blue colors. Manganese produces reds and pinks, and possibly yellows. 

Unpolished tourmaline crystals are often just as beautiful as fashioned tourmalines.Well-formed tourmaline crystals are very distinctive. They’re usually elongated with a rounded triangular cross-section.They have fine grooves— or striations—along their length.  Cutters often fashion tourmalines as long rectangles. Making the cut parallel to the length of the rough crystal helps to reduce waste. But cutters also have to consider tourmaline’s optical properties. 

Tourmaline is strongly doubly refractive and pleochroic. Its pleochroism is even stronger than the pleochroism of zircon and peridot. One of tourmaline’s pleochroic colors is typically much darker than the other. In addition, many tourmalines absorb more light down the length of the crystal than across it. 

Rather than cutting every tourmaline lengthwise, many cutters orient a fashioned gem based on its depth of color. To darken pale rough, they might orient a gem’s table so that it’s perpendicular to the crystal’s length. To lighten dark rough, they orient a finished gem’s table so that it’s parallel to the crystal’s length. In that case, you’ll see both pleochroic colors through the table when you examine the gem with a dichroscope.

Colored tourmalines grow in an environment rich in liquids, and some of those liquids are often captured as inclusions during crystal growth. The most typical inclusions resemble thread-like cavities that run parallel to the length of the crystal. Under magnification, you can see that they’re filled with liquid or gas bubbles.These inclusions are also known as trichites. Growth tubes—long hollow tubes often capped with minute mineral crystals—are also common tourmaline inclusions. If they’re numerous enough, and the rough is correctly cut, they can cause a cat’s-eye. 

Cat’s-eye tourmaline owes its phenomenon to groups of parallel inclusions. The inclusions in tourmaline are fairly large, hollow tubes rather than fine needles. Because they’re bigger in diameter than the needles or growth features in chrysoberyl, they create a coarser cat’s-eye. So even in fine-quality cat’s-eye tourmaline, the phenomenon appears less focused than that in good-quality chatoyant chrysoberyl. The most common colors for cat’s-eye tourmaline are various shades of pink, red, green, blue, and greenish blue. To reveal the cat’s-eye, cutters fashion suitable crystals as cabochons.

Brazil is a major source of tourmalines in almost all colors. Other important sources are Nigeria, Zambia, Mozambique, Madagascar, Namibia, Tanzania, Kenya, Afghanistan and Pakistan. Tourmaline deposits are also found in Russia and the US. Mines in San Diego County, California. 

Paraíba is a trade term for rare, expensive, vibrant green to blue or violet elbaite tourmaline, originally found only in Brazil’s Paraíba state. Overall, prices for the best Paraíba tourmalines easily surpass other tourmalines due to their more attractive hues, higher color saturation, and greater rarity.

The two most important tourmaline treatments are heating and irradiation. Many green and blue tourmalines are heat-treated after they’re cut and polished.Treatment usually lightens and brightens the original color rather than changing its hue. Gems with abundant liquid inclusions can’t withstand heat treatment, so if you notice liquid inclusions within rubellite, it’s a fairly sure sign the gem hasn’t been heated. 


Source: Gemological Institute of America

Photograph: Dichroscope, used for viewing a gem's pleochroic colors
© Artin Boghosian