studies of precious and common opal go back
at least to the first half of the nineteenth
century. However work on poorly crystalline
silicas occurring in nature was intermittent
and comparatively few papers were published.
Most of the work on this subject has been
done since the early 1960's, when the initial
discoveries of workers in CSIRO (Commonwealth
Scientific and Industrial Research Organization)
and The University of Adelaide in Australia
gave an impetus to the study of natural
opaline materials. Somewhat later, a further
impetus to these studies was given by the
awakening of interest in the importance
of the silica cycle in nature, both on land
and in the oceans.