Some minerals change colour when exposed to radiation
So some coloured minerals are, in effect, "radiation detectors" -
they tell us they were once exposed to radiation.
The coloured irradiated minerals are generally not radioactive
However be cautious - the rocks that contain the minerals may be
Elements are made up of protons, neutrons and electrons. Put very simply -
radioactive elements contain too many bits inside to remain stable so from
time to time they get rid of bits - this is called radiation.
The colour of smoky quartz is caused by irradiation and traces of
aluminum built into its crystal lattice.
In normal geological environments this process can only take place at
temperatures below 50°C
True smoky quartz will loose its colour when heated to about 200°C, and
the colour will occur again when the crystal is irradiated with x- or
gamma-rays. Some (but reportedly not all) smoky quartz is sensitive to
ultraviolet light and will pale when exposed to sunlight for a long time
It's still common practice to artificially irradiate colourless quartz and
sell it as smoky quartz.
There is no radiation hazard associated with irradiated smoky quartz.
Irradiating a crystal will cause some energy to be stored in the crystal
structure, which can change the optical properties, typically observed as
a change in colour.
Using radiation to change the colour of gemstones has been studied almost
as long as radioactivity has been known. An early study, published within
10 years of the discovery of radioactivity, placed diamonds in radium
powder to give a green colour. As the use of radiation has become more
sophisticated, the ability to treat gemstones with radiation in order to
create colour changes has become more common.
Some minerals that are "radiation dectectors include...
Fluorite is irradiated to intensify its colour or induce colour
(notably blue), usually by gamma rays from Cobalt-60 source.
Beryl - beryl is heated to remove the golden to green shades
that result from radiation in oder to turn the crystal into blue
Calcite - Radiation is associated with blue and amber colours
Diamond - Naturally occurring green diamonds are coloured by
natural radiation, yellow diamond, 47K; A technological product of
irradiation and heating. orange diamond, 53K; A technological product of
irradiation and heating. blue diamond, 29K; A technological product of
irradiation. Although they look similar to natural blue diamonds, the
colour is from an entirely different origin. green diamond, 68K; A
technological product of irradiation.
Halite (salt) - blue halite from Germany is the result of
exposure to natural radiation
Quartz - will also turn purple and is known as amethyst is the
result of natural irradiation of Fe3+ in the quartz to Fe4+. The purple
amethst colour will fade in time when exposed to sunlight
Topaz - turns a brown colour but this colour is unstable and
fades in light in a matter of hours to days Halite (salt) turns blue
Calcite turns honey brown