Abstract
Precious and common opals from the Hosaka mine, Fukushima Prefecture, Japan were studied by means of optical and transmission electron microscopies. The precious opal showing weak play of color of red, green and violet consists of amorphous silica sphere, whose diameter is from 400 to 500 Å. Some spheres elongate slightly in one direction, and their packing is not regular. The origin of the play of color in this opal cannot be explained by diffraction of light in regular packing of silica spheres. Commom opal, whose x-ray diffraction pattern corresponds to opal-CT (Jones et al. 1971), consists of short or long acicular crystals which have nearly constant diameter about 200 Å but vary in length from 1000 to 100, 000 Å in length. The electron diffraction patterns show these crystals to be disordered high tridymite.
