Opal (noun)

1. A precious stone consisting of hydrated silica of variable colour, which exhibits a play of colours like that of the rainbow.
2. A mineral consisting of hydrated silica, SiO2·nH2O, that is usually opaque but can be transparent in thin sections and that typically exhibits a play of colors caused by the interference and diffraction of light by internal microstructures.

Origin:

Middle english: from old french opale, via latin from greek opallios "a precious stone".

Examples:

1. She wore a beautiful opal ring on her finger.
2. The opal is a rare and valuable gemstone.
3. Opals are said to bring good luck.
4. The opal was said to change colour depending on the light.
5. Opals are found in many parts of the world, but the most famous ones come from Australia.