Its blue color ultramarine is obtained by irradiation.
The blue London Blue is generally obtained by bombardment of electrons, followed by heating to stabilize the color
In answer to the expressed fears, there is a very strict legislation which was organized by the laboratories which handle Topazes. These are stored before being put into service. Stones on sale do not have persistent irradiations anymore. Think of the jewelers or the lapidaries who spend their time with these stones in hands and who are not irradiated for all that!
The blue London Blue is generally obtained by bombardment of electrons, followed by heating to stabilize the color.
To know more about the mineralogy of the Topaz London blue, see mindat.org ->
