Summary: Gold appears to be even more widely distributed in minute amounts in minerals and rocks than has hitherto been generally recognized. It is emphasized that many factors may be involved in producing the colours of minerals. The mere presence of gold is not in itself proof that it is responsible for the colour. Nevertheless, reasons are given for the suggestion that the translucent blue tints of the specimens of celestine, anhydrite, and halite could be caused by colloidal gold. The translucent pink and reddish-brown tints of the halite and celestine specimens examined may likewise be connected with their gold contents. Gold has been found in several other minerals, but not in a natural ruby. Aragonite from Lunehead mine, Yorkshire, was tinted with copper and bore a close resemblance to mossottite from Tuscany.
The authors desire to express their deep indebtedness to the various institutions and persons mentioned in the table who have so generously supplied them with the mineral specimens described.