Watermelon Tourmaline Buying Guide
How Does Watermelon Tourmaline Form?
Rough tourmaline crystals form in elongated shapes underground. To be a true “watermelon,” this crystal should have a pink or red core with a green rind. The pink or red center of these crystals forms first. Then, a shift in chemistry in the geothermal fluid that creates the crystal occurs. Instead of manganese, which causes the pink color, iron enters the solution, which imparts a green hue to the crystal.
While bi-color and parti-color tourmalines with color transitions along the longer axis of the crystal also form, these aren’t considered watermelon tourmaline. Nonetheless, some dealers refer to any bi-color pink and green tourmalines as watermelon tourmaline.
Watermelon Tourmaline Buying and the Four Cs
The IGS tourmaline value listing has price guidelines for tourmalines.
By far, the most important criterion for assessing the value of watermelon tourmaline is color. Traditionally, this gem has a pink center with a ring of green around the edge. Sometimes, a colorless ring separates these colors. “Reverse” watermelon tourmalines, with a green core and pink edge, also occur. In addition, because tourmalines can form in any color of the rainbow, other color combinations may occur. While these aren’t traditional “watermelons,” a specimen with these rarer colors may fetch a good price.
To grade color in watermelon tourmalines, consider each color separately. A saturated, deep red or pink is more valuable than a lighter tone. The green-hued area should also be saturated and may even reach darker tones than the pink or red. Each hue should be distinct, with no zoning in tone. Ideally, the transition between hues is abrupt, with no fading or brown zone between colors (though a colorless zone between green and pink is common). The proportion of pink area to green rind also affects value. A more watermelon-like proportion is desired, with a relatively thin green area around the pink core.
Because watermelon tourmalines grow in an area of changing water chemistry, a specimen with good clarity is rare. Imperfections are tolerated, especially in the color transition zone. In addition, black inclusions on the pink area of the stone are highly salable for their resemblance to watermelon seeds.
While watermelon tourmalines are commonly sold as slices, some lapidaries create cabochons. Cabochons may be especially attractive options for specimens with a lighter center color and darker rim, as the shape can deepen the core and lighten the edges somewhat. Faceted rose cuts are also available.
Price per carat doesn’t vary much for this gem, making it an affordable choice even in larger sizes.
While tourmaline is a tough gem that resists scratching, watermelon tourmalines may be susceptible to breaking. Surface-reaching inclusions or fractures can weaken the gem, making it more likely to break. Gems set in rings and bracelets are likely to experience accidental knocks, so this gem may be best in a pendant or earrings.