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optical microscope
unexpected structure


Scientific 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.