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Minerals which have evenly spaced inclusions of other minerals, or minute cracks, or are intricately twinned, can reflect and bend light in such a way as to produce a peculiar, but for some minerals characteristic, play of colours. The variety of plagioclase called labradorite, for example, often shows a rich play of colours in which blue and green predominate. The colours of labradorite change when the crystal is viewed from different angles, the variation in colour being caused by the interference of light reflected from minute, repeatedly twinned plates of the mineral which are intricately intergrown, as well as from regularly-spaced lamellar inclusions of haematite, magnetite or ilmenite.

Opalescence is the term applied to the play of colours characteristic of opal, and can be attributed to the peculiar internal structure of the mineral through which the light is reflected. When opal is turned around or viewed from different directions, it produces a play of moving rainbow colours on a milky white background.
Iridescent colouration is seen on crystals whose surface is tarnished by exposure to the air. The tarnish is usually due to oxidation or chemical action by other gases, such as sulphur dioxide, in the atmosphere. Tarnish can appear on a large number of minerals and may sometimes lead to some confusion in their identification. Minerals which tarnish very readily are covelline and erubescite. The latter is often called peacock ore.