Watermelon tourmaline is a variety of concentrically color-zoned tourmaline with red interiors and green exteriors and is distinct from longitudinal bi-color or polychrome zonation. The name was coined by George Robeley Howe [1860-1950] of Norway, Maine, USA. The name was first used in a Lewiston, Maine newspaper account in 1910 when describing specimens from the Havey Quarry in Poland, Maine, USA. The name was used in an international publication, Mineral Resources of the United States, by Douglas B. Sterrett in 1911.

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.

Metaphysical Properties of Watermelon Tourmaline: Emotionally healing, "super activator" of heart & higher chakras, clears blockages in giving or receiving love, balances yin and yang / physical and spiritual aspects of self, calms emotions, encourages connection to nature and connection to self-love, increases empathy and compassion, protection against negativity, letting go of past patterns

Source:
https://www.gemsociety.org/article/watermelon-tourmaline-buying-guide/
https://www.mindat.org/min-10889.html