Tourmaline has become a favourite gemstone among jewelry designer, and gem collectors the world over. Since it is available in a wide variety of colours, it is ideally suited to almost anyone’s taste. Tourmaline also is known for displaying several colours in the same gemstone. These bi-colour or tri-colour gems are formed in many combinations; gemstones with clear colour distinctions are highly prized. One multi-colour variety is known as watermelon tourmaline, and features green, pink, and white colours bands; to resemble its namesake, the gemstone is cut into thin slices having a pink center, white ring, and green edge. Tourmaline is found in many localities including Brazil, Afghanistan, East Africa, and the USA.
The name opal derives from the Greek Opallos, meaning “to see a change (of colour).” Opals range in colour from milky white to black with flashes of yellow, orange, green, red, and blue. An opal’s beauty is the product of contrast between its colour play and its background. Opal is a formation of non-crystalline silica gel that seeped into crevices in the sedimentary strata. Through time and nature’s heating and moulding processes, the gel hardened into the form of opals. The opal is composed of particles closely packed in spherical arrangements. When packed together in a regular pattern, a three-dimensional array of spaces are created that give opal its radiance.
Information from http://www.americangemsociety.org