TOURMALINE

GEMOLOGICAL PROPERTIES OF TOURMALINE

Chemical Composition:

Tourmaline is a complex borosilicate mineral with a variable composition. The general formula for tourmaline is (Na, Ca)(Li, Mg, Al, Fe, Mn)(Al, B, Si)₆O₁₈(BO₃)₃(OH, F)₄. The specific composition can vary depending on the tourmaline variety and its color.

Color:

Tourmaline is known for its wide range of colors. It can be found in virtually all colors of the rainbow, including pink, red, green, blue, brown, and black. Some tourmalines also exhibit multiple colors in a single crystal, a phenomenon known as bicolor or multicolor.

Color Zoning:

Tourmalines may display color zoning, where different colors are present in distinct zones within the same crystal. This can affect the overall appearance and value of the gem.

Varieties:

Common varieties of tourmaline include:

Elbaite:

This is the most common variety and is responsible for the wide range of colors.

Rubellite:

Refers to red or pink tourmaline.

Indicolite:

Denotes blue tourmaline.

Verdelite:

Represents green tourmaline.

Watermelon Tourmaline:

Displays a pink or red center surrounded by a green outer rim, resembling a watermelon.

Cut:

Tourmalines are often faceted to enhance their brilliance and color. Cabochon cuts are also popular, especially for varieties like watermelon tourmaline.

Clarity:

Tourmalines can range from transparent to opaque. Inclusions are common, and some varieties may exhibit a cat’s eye or asterism effect.

Mohs Hardness:

Tourmaline has a hardness of 7 to 7.5 on the Mohs scale, making it relatively durable and suitable for various types of jewelry.

Luster:

The luster of tourmaline is typically vitreous (glass-like), but it can vary depending on the specific variety and the quality of the cut.

Specific Gravity:

The specific gravity of tourmaline ranges from 3.02 to 3.26, providing a useful property for gemstone identification.

Dispersion:

Tourmaline has low to moderate dispersion, meaning it doesn’t exhibit the same fire as some other gemstones like diamond.

Piezoelectric Properties:

Tourmaline is piezoelectric, meaning it can generate an electrical charge when subjected to pressure. This property has led to various technological applications.

Origin:

The geographic origin of tourmaline can influence its value. Some famous sources include Brazil, Afghanistan, Nigeria, Madagascar, and the United States.

Treatment:

Heat treatment is commonly used to enhance or alter the color of tourmalines. Some varieties may also undergo irradiation to produce certain colors.

Understanding these gemological properties is important for gemologists and buyers when assessing the quality and value of tourmaline gemstones.