Some
other phenomena in relation to light remain to be noticed. Some
minerals, especially Labradorite, when viewed in certain directions are
seen to give a brilliant colour effect. This is in no way due to any
material pigment, but wholly to the minute structure of the stone
causing an optical colour effect, as is caused in the feathers of many
birds by their structure. This effect is known as change of colour.
Iridescence is caused in a somewhat similar way by interference of
light in minute air-filled cracks in the mineral. It is often seen from
natural flaws in Quartz crystals, and is sometimes intentionally
produced by suddenly cooling a heated piece of this mineral. The colour
of Opal is sometimes ascribed to this cause, and sometimes referred to
the bending and dispersion of the light rays. Minute crystalline
structure may also produce the quiet soft change of light seen in the
Adularia or Moonstone so well. The same effect occurs even more
markedly in some specimens of Chrysoberyl, causing a streak of light to
appear on turning the stone, which is then, from its appearance, called
Cat's Eye. Other minerals showing this change are known under the same
name; for example, some specimens of Quartz, though in this case it is
due rather to a fibrous structure than to a minute platelike
arrangement. The appearance in this case is called chatoyancy.
Asterism,
so often seen in crystals of Sapphire cut at right angles to the
vertical axis of the crystal, is due either to minute canals crossing
in one plane at angles of 120°, or to an optical effect from
twin-plates in the crystal. Fluorescence is a change of colour in a
mineral, as seen when viewed first by reflected and then by transmitted
light.
