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This matched pair of iris watermelon elbaite tourmaline slices is from Minas Gerais, Brazil. The iris effect is due to small fractures in the stones. Credit: Russ Behnke

Watermelon tourmaline (primarily elbaite and/or liddicoatlite) is a variety of concentrically color-zoned tourmaline with red interiors and green exteriors and is distinct from longitudinal bi-color or polychrome zonation.

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 others 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.

How Does It Grow?
  
Juicy watermelon tourmaline from Brazil. Photo: Julia Aufenast

Tourmaline is produced in pegmatites. These are voids deep under ground that are filled with a rich hot liquid that is rich in different minerals. When the liquids begin to cool, the minerals crystallize and form Tourmaline crystals. The host rock around these pegmatites is generally granite or schits. Some species will also grow in metamorphic rock such as marble.




Tourmaline is found in granite and granite pegmatites and in metamorphic rocks such as schist and marble. Schorl and lithium-rich tourmalines are usually found in granite and granite pegmatite. Magnesium-rich tourmalines, dravites, are generally restricted to schists and marble. Tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate, and is part of the ZTR index for highly weathered sediments.

Crystals can either grow with a central core of one color and outer rim of another color, or with bands of different color along the crystal shaft. Any combination of colors is possible. The most frequently seen bi-color gem is “watermelon” Tourmaline, in which pink or red color meets green color. Bi-color Tourmalines can show different magnetic responses within the same gem, with pink/red sections tending to be the most weakly magnetic, or diamagnetic.

Since they are formed in a liquid rich environment, the inclusions can contain small amounts of liquid trapped inside the stone.
 
Usually, iron-rich tourmalines 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.

Gorgeous Watermelon Tourmaline FROM Brazil.
specimen: Fine Mineral International, photography: James Elliott




Localities for Watermelon Tourmaline
  1. Brazil (Minas Gerais,Coronel Murta,Barra do Salinas) 
  2. Madagascar (Antananarivo Province,Vakinankaratra Region, Sahatany Pegmatite Field (Mt Ibity area),Sahatany Valley) 
  3. USA 
    • Arizona, Maricopa Co.
    • California, Riverside Co.,Cahuilla District,Thomas Mountain [town],Thomas Mountain).
    • California, San Diego Co.,Mesa Grande District,Gem Hill)
    • California, Pala District,Pala,Hiriart Mountain (Hariat Mtn; Harriot Mtn; Heriart Mtn; Heriot Mtn; Hiriat Hill).
    • Colorado, Fremont Co.,Eight Mile Park pegmatite District.
    • Maine, Androscoggin Co.,Auburn,East Mount Apatite District.
This is the famous "Steamboat" tourmaline mineral specimen from the Tourmaline King mine in the Pala District of San Diego County, California. Photo by Duncan Pay

References:

Gemstone Magnetism
Wikipedia
Mindat
AJS Gems

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