Tourmalines are a group of minerals from the class of silicates. Due to the wide variety of colors and a Mohs hardness of 7 to 7.5, these are popular stones for the jewelry industry. Zonal colored tourmalines are especially sought after. Pink to red tourmalines are called rubellite, blue indigolite and green verdelite. Pink to red tourmalines with a green edge are reminiscent in color of a cut open watermelon and are therefore called melon tourmaline.