Opal may exhibit a spectrum of colors called play-of-color. When this is set against a dark background, it is known as black opal. Australia is the source of most of the world’s fine opal. Opal represents hope.
Tourmaline exhibits a wide range of colors. Bicolored and multicolored crystals are common. Sometimes crystals are green on the outside and pink inside; these are called watermelon tourmalines. Wearing tourmaline is believed to bring strength.
Source: GIA Gem Encyclopedia