Tourmaline Description

They're often brown, yellowish brown, reddish brown, or nearly black in color, but sometimes they contain traces of vanadium, chromium, or both. When present in ...

Bi-Color, Tri-Color, Multi-Color or Watermelon are terms used for Tourmalines with 2 or more colors. Significant changes going on in the environment where these crystals grew explain the distinct or subtle changes in color along the length of the crystal.

Multi-colored tourmaline is found in combinations like gold with pink and red, or green, gold and red. It's called by many names, parti-colored, bi-color, tri .

Tourmaline is a six-member ring cyclosilicate having a trigonal crystal system. It occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section, often with curved striated faces. The style of termination at the ends of crystals is sometimes asymmetrical, called hemimorphism. Small slender prismatic crystals are common in a fine-grained granite called aplite, often forming radial daisy-like patterns. Tourmaline is distinguished by its three-sided prisms; no other common mineral has three sides. Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely perfectly euhedral. An exception was the fine dravite tourmalines of Yinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals are piezoelectric, and are often pyroelectric as well.

Tourmaline has a variety of colors. Iron-rich tourmalines are usually 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 and is prized in jewelry. An excellent example of watermelon tourmaline jewelry is a brooch piece (1969, gold, watermelon tourmaline, diamonds) by Andrew Grima (British, b. Italy, 1921–2007), in the collection of Kimberly Klosterman and on display at the Cincinnati Art Museum.[19] Some forms of tourmaline are dichroic; they change color when viewed from different directions.[20]

The pink color of tourmalines from many localities is the result of prolonged natural irradiation. During their growth, these tourmaline crystals incorporated Mn2+ and were initially very pale. Due to natural gamma ray exposure from radioactive decay of 40Kin their granitic environment, gradual formation of Mn3+ ions occurs, which is responsible for the deepening of the pink to red color.