A brief review of structure and formation of pearls is given from the viewpoint of biomineralogy. Pearls are a gem produced by the metabolism of molluscs and are composed of nacreous layer containing more than 90% of calcium carbonate and about 5% of protein. The outer epithelial cells of the mantle piece, which are transplanted into the gonad or mantle tissue of another individual of the same species together with a pearl nucleus or without the nucleus, regenerat a pearl sac consisting of a single layer of cells and secrete inside the sac. Pearl mineral, tabular crystal of aragonite, grows alternatively with protein sheet in the secretion to form the laminar structure of nacreous layer which possesses a characteristic pearly iridescence. The mineralization of pearl and chemical composition of secretion and pearls are explained in this review.

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In this part, recent results of optical and electron microscopic investigations of opals are reviewed, together with explanation of geological settings of opal localities, so as to throw light to the understanding of the origin of play of colour and for the elucidation of the genesis of opals. On the fracture surface of precious opal with play of colour, wavy or circular banding patterns are seen under the reflection optical microscope, and they are inferred to the phenomenon of optical diffraction due to regular packing of silica spheres. Such banding patterns are seen only where play of colour is seen. Simple experiments, however, demonstrated that the banding patterns correspond to the steps exposed on the fracture surface, and not to optical diffraction. Under the electron microscope, it is observed that they are steps formed between different lamellae consisting of silica spheres of nearly equal sizes. Electron microscopic observations of both fresh and etched fracture surfaces revealed that there are three different types of lamellae ; 1) those without voids, 2) those consisting of unit layer without voids and with three voids around silica spheres, 3) those consisting of unit layer with six voids and with three voids. Opal can be classified as shown in Table 2, depending on the existence and types of voids. The banding patterns vary depending on this difference. Based on these observations, the origin of play of colour is discussed. Geological occurrences of precious opals in Australia, Mexico, Czecoslovakia, and Japan are described in some details, and the processes of opal formation both in sedimentary and igneous environments are discussed.

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The basic knowledge on the studies of fluid inclusions in minerals is introduced as a basis for gemmology. The definition and identification of primary, secondary and pseudosecondary inclusions, and their significance for the condition of crystal growth are discussed briefly. The classification of fluid inclusions based on the phase assemblages is also described. They are calssified into five groups, that is, gaseous inclusion, liquid inclusion, polyphase fluid inclusion. CO_2-bearing fluid inclusion and petroleum-bearing fluid inclusion. The mechanism of trapping of primary and secondary inclusions is explained. Microphotographs of some typical fluid inclusions such as gaseous, liquid, polyphase and CO_2-bearing inclusions are shown.

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