『Abstract
　The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide localities was investigated using scanning electron microscopy (SEM). These opals do not show play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and opal-CT are primarily built from nanograins that average 〜25 nm in diameter. Only opal-AN has a texture similar to that of glass. In opal-AG, nanograins arrange into spheres that have successive concentric layers, or in some cases, radial structures. Common opal does not diffract light because its spheres exhibit a range of sizes, are imperfectly shaped, are too large or too small, or are not well ordered. Opal-AG spheres are typically cemented by non-ordered nanograins, which likely result from late stage fluid deposition. In opal-CT, nanograins have different degrees of ordering, ranging from none (aggregation of individual nanograins), to an intermediate stage in which they form tablets or platelets, to the formation of lepispheres. When the structure is built of lepispheres, they are generally cemented by non-ordered nanograins. The degree of nanograin ordering may depend on the growth or deposition rate imposed by the properties of the gel from which opal settles, presumably, fast for non-ordered nanograin structures in opal-CT to slow for the concentric arrangement of nanograins in the spheres of opal-AG.

Keywords: Opal-A; opal-CT; common opal; structure; SEM; nanograin』

Introduction
Background
　Structure of opal-AG
　　Cementation and section-orientation effects
　　Internal structure of the spheres
　Structure of opal-CT
Materials and experimental methods
　Material
　Experimental methods
Results
　Structure of common opal-AN
　Structure of common opal-AG
　　Arrangements of spheres not giving rise to play-of-color
　　Internal structures of spheres
　　Cementation and section-orientation effects
　　Compaction of the spheres
　Structure of common opal-CT
　　Random aggregation of individual nanograins
　　Nanograins arranged in fibers
　　Nanograins arranged in platelets
　　Nanograins arranged in lepispheres
　　Section-orientation effects
Discussion
　Common and play-of-color opal: Where to draw the line?
　The nanograin is the elementary building block of all opals, except opal-AN
　Influence of growth rate on opal structures
　Imperfect conditions lead to an imperfect network, thus to common opal
　Cementation
　Formation environment for gem common opal-AG and opal-CT
　Future work
Acknowledgments
References cited

opal-A	amorphous
	opal-AN	(network), or “hyalite,” which shows only diffuse scattering of X-rays or neutrons at small angles, suggesting that it has a glass-like structure	Langer and Florke（oの頭に¨）(1974)
	opal-AG	(gel), which is the most widespread variety. In small-angle X-ray or neutron patterns, it exhibits obvious intensity maxima superimposed upon the diffuse scattering, indicating a structure consisting of packed silica spheres. The term opal-A typically is synonymous with opal-AG.
opal-CT		cristobalite-tridymite, which consists of disordered α-cristobalite with tridymitic stacking
opal-C		which the proportion of cristobalite much greater than that of tridymite