Note on the colour of zircons, and its radioactive origin

Crystals of the mineral zircon are found in a variety of different colours. What I have to communicate refers to the brown kinds. Of these, two may be' distinguished: the common opaque brown variety, easily obtained in large quantity from the South of Norway and from North Carolina, and the transparent reddish brown kinds known as hyacinth, and obtained, for instance, from Expailly in Auvergne, from Unkel, on the Rhine, and from Campbell Island, New Zealand. It is remarkable that the opaque kinds occur in plutonic rocks, such as syenites, and the transparent (when their matrix can be traced) only in basalts and other lavas. Their outline is rounded, even when found, as at Unkel, embedded in perfectly fresh basalt. For this reason it has been supposed (and I have no doubt correctly) that such zircons have been derived from the melting down of plutonic rocks which originally contained them. Zircon, from its extreme resistance to chemical attack, would survive almost all other minerals. Incipient chemical attack would account for the rounded shape, which is in extreme contrast to the sharp crystal outline of the zircons in their original home in plutopic rocks. Hyacinths lose their reddish brown colour completely when heated to a temperature of about 300°C., and this fact, considered in relation to their occurrence embedded in a solidified lava, presents at first sight no small difficulty. How is it that they were not decolorised by the heat of the molten rock ? Or, if they were decolorised, how has the colour been recovered ? For it will hardly be suggested that transparent crystals of a material so resistant have been coloured by any action of percolating water