Tourmaline is the name for a group of gemstones that are all made up of the same crystalline structure. They come in various colours.
The pink and red variety of tourmaline is called Rubellite. There are green and yellow tourmalines and even watermelon tourmalines with a pink core and green outer edge (or vice versa): exactly the colours of the red flesh and the green peel of a watermelon. There are many tourmalines comprised of two colours. Hence the name, which is derived from the Senegalese word Turmali, which means something akin to ‘stone with the mixed colours’. The tourmaline shown in the picture is called a watermelon tourmaline.