What are watermelon tourmalines?
All gems in the tourmaline family are mixed crystals of aluminium boron silicate that also contain elements such as iron, manganese, sodium, lithium, or potassium. While tourmaline was first discovered in Brazil in the 16th century, the green crystals were initially confused with emerald. It was not until the 19th century that the gem was classified as tourmaline.
Gemmologists now divide tourmalines into 11 different species depending on their properties and chemical composition. Elbaite is the name given to the most colourful members of the tourmaline mineral family. The red or pink tourmalines known as rubellites, the blue variety called indicolite and the coveted Paraiba tourmaline are all Elbaites, as is the multi-coloured watermelon tourmaline.
The term tourmaline was derived from the Singhalese phrase tura mali, meaning “stone mixed with vibrant colours”, and the watermelon prefix refers to the unique colour combination that resembles the fruit of the same name. With a Moh’s scale hardness of 7-7.5, the watermelon tourmaline is a relatively durable stone for jewellery while its distinctive and eye-catching colours mark it out as a favourite with designers.
How is watermelon tourmaline formed?
Changing conditions during tourmaline crystal growth often result in single crystals that contain two or more different colors of tourmaline. The earlier color is usually overgrown by the later color. These bicolor crystals are known as “zoned crystals.” Cut gemstones with distinctly different color zones are known as parti-color gems.
In many gems, color zoning is undesirable because most gem and jewelry buyers prefer stones that have a single, uniform face-up color. Tourmaline is an exception to this trend. Gems cut from color-zoned crystals with pleasing colors are a novelty prized by designers and collectors.
Color-zoned crystals are often sawn into thin cross-sections and polished. These thin bicolor gems can be very attractive. The most popular bicolor tourmaline is “watermelon tourmaline.” It has a pink interior and a green rind – just like a slice of watermelon. The closer the colors match those of a real watermelon, the more people enjoy them and the higher the price.
Tourmaline crystals are also faceted to produce bicolor gems. “Watermelon” is again the most popular, but many other beautiful color combinations are cut.
Zoned tourmaline crystals often have clarity problems in the color-change area. If the color combination is attractive, minor clarity problems usually do not have a major impact on their desirability or price.
Where is watermelon tourmaline found?
Brazil, Nigeria, Madagascar, Afghanistan and the United States all produce watermelon tourmalines. The North American state of Maine, where the gem was first discovered at a mine in Newry in 1902, is renowned for its stunning watermelon tourmaline. In the early 1970s, a large pocket of some of the finest watermelon tourmalines ever seen was unearthed at the nearby Plumbago Gem Pit.
Brazil is also famous for its gem-quality watermelon tourmalines, with amazingly clear and vibrant colours. Gem dealer Matthew Morrell of Direct Source Gems says the most abundant sources of classic red/green watermelon tourmalines are the Araçuaí and São José da Safira regions of Minas Gerais in Brazil.