Description

This is a crystal cleavage displaying an amazing phenomenon sometimes observed in halite – it exhibits intense blue internal colour that arises from having been exposed in the past to natural gamma radiation. In this case, the radiation is from Potassium-40, present in the sylvite that surrounds the halite at this deposit. The radiation causes structural defects in the crystal lattice (at a subatomic level). Displaced electrons migrate within the crystal lattice, attracted to positive sodium ions, forming sodium metal. Atoms of sodium metal, in turn, migrate to form colloidal sized aggregates of sodium metal. These sodium metal aggregates within the crystal structure cause the blue colour. (http://minerals.caltech.edu/COLOR_Causes/Radiate/index.htm)

This specimen from the Pete Richards collection shows the internal colour beautifully!

NB: do not expose to water.