Tourmaline consists of a group of isomorphous minerals with an identical crystal lattice. This means that each type of gemstone is made up with different chemicals, but has the same physical structure.
It can occur in any colour, and stones that are part one colour and part another are fairly common. One example of this is watermelon tourmaline, which contains both pink and green parts to a single stone.
Heat treatment or irradiation is frequently used to enhance the colour.
Tourmaline is one of the birthstones for October.