DIFFERENT TYPES OF TOURMALINE

Saturday, 30.Sep 2023

As gemstones form in nature, very often imperfections can occur within them. These can take many forms including cracks due to high pressure.

THE MEANING OF TOURMARLINE

Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium or potassium. It is hard and durable and very well suited for jewelry.

Tourmaline is classified as a semi-precious stone and the gemstone comes in a wide variety of color. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles.

Through Elbaite occurs in all color forms, the term Elbaite in the gemstone trade is sometimes used to describe the green form of Tourmaline, the other color forms of elbaite have their own variety names on the gem market. Traditional Tourmaline gemstone variety names include rubellite, the red or pink variety, indicolite, the blue variety and watermelon tourmaline, a multicolored tourmaline of green and red.

THE COLORS OF TOUMALINE

Some chrome tourmaline display a color change. This term usually signifies a color difference when a stone is moved from daylight into incandescent light, such as candlelight, Alexandrite, for example is famous for its change from green to red when this is done. The effect seen in tourmaline is more complicated. There is a tendency for red to be seen more easily in incandescent light and green in daylight, but the color change depends mostly on the thickness of the stone. Thin stone look green when light is shone through them, orange and then red are seen as thickness. The thickness required depends on the intensity of color, but about 15mm is enough to cause many stones to look red.

Color zoning is a common feature of tourmaline, some crystals are zoned concentrically from their centres to their surfaces so that the inner parts of polished cross sections have different colors from the outer parts, when a green border surrounds a pink core, the stones are called watermelon tourmalines, but has sometimes been extended to other color combinations.

PARTI-COLOURED TOURMALINE

Parti-colored tourmaline is any stone in which this kind of color variation occurs, but also refers to stones that have different color along their length. In this case, the end of crystal is one color, while the base, is another color.

Tourmaline color have many different causes, usually, iron-rich tourmaline are black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are almost any color: blue, green, red, yellow, pink, etc.

Rarely, it is colorless. Bi-colored and multicolored crystals are common, reflecting variations of fluid chemistry during crystallization. Crystals may be green at one end and pink at the other, or green on the outside and pink inside; this type is called watermelon tourmaline. Some forms of tourmaline are dichroic, in that they change color when viewed from different directions

All colored tourmaline gems display pleochroism, meaning their color changes when viewed at different angles. In some tourmaline gems, this effect is hardly noticeable, while in other it is strongly apparent. Gemstone cutters must take this into account when cutting a tourmaline, so that the finished gem brings out its best color

The most expensive and valuable form of tourmaline is the rare neon-blue form known by the trade name Paraiba Tourmaline. Paraiba tourmaline was first discovered in gem pegmatite in the Brazilian state of Paraiba in 1989.

Tourmaline is found in granite and pegmatites and in metamorphic rocks such as schist and marble. Schorl and lithium-rich tourmalines are usually found in granite pegmatite. Magnesium-rich tourmalines, dravites, are generally restricted to schists and marble.

THE COMMON SPECIES AND VARIETIES OF TOURMALINE

Schorl

The most common species of tourmaline is schorl, the sodium iron (divalent) endmember of the group. It may account for 95% or more of all tourmaline in nature. Opaque black school and yellow tsilaisite are indiochromatic tourmaline species that have high manganese respectively. Schorl is exclusively black and unlike other tourmaline forms, is never transparent or even translucent. Schorl is the most common form of tourmaline. It usually elongated prismatic crystal hat are heavily striated. Also as short, stubby, prismatic crystal.

Dravite

Dravite, also called brown tourmaline is the sodium magnesium rich tourmaline endmember. Dravite varieties include the deep green chromium dravite and the vanadium dravite.

Liddicoatite

Liddiciatite was not recognized as a tourmaline species until 1977. Prior to that time, it was thought to be Elbaite. The color is usually smoky brown, but also pink, red, green, blue, or rarely white. Color zoning is abundant at the type locality, parallel to pyramid faces. This is due to changes in the solution during crystal growth.

Elbaite

Elbaite is the most well-known and valuable form of tourmaline. Most of the multicolored tourmaline and almost all of the tourmaline gemstone are of the Elbaite variety. Elbaite is allochromatic, meaning trace amount of impurities can tint crystals, and it can be strongly pleochroic. Every color of the rainbow may be represented by elbaite, some exhibiting multicolor zonation.

Microscopic acicular inclusion in some elbaite crystals show the cat’s eye effect in polished cabochons.

CARE

Tourmaline is very wearable with a Mohs harness of 7 – 7.5. its easy to wear and easy to care for. Avoid steam or ultrasonic cleaning and avoid sudden temperature changes. Tourmaline will respond well to warm, sudsy water, a thorough rinse and soft-cloth dry. Avoid harsh chemicals; use only gentle soap.

LIGHTING FACTOR & DISCHROISM