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OPAL. —PEARL.
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28 Γ
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Newton's theory has its objectors, some of whom argue that
the beautiful, variegated reflections which lend such a charm to this gem are caused by lamina;, a structure found in some other minerals. Whatever may have produced the exquisite tints, it is conceded that its special attraction is due to its imperfections, and not to any coloring matter, as in the case of most other precious stones. In the limpid varieties, it has been stated, it is difficult toperceive any cause for the play of colors seen in this gem, but
in the translucent specimens the case is entirely different. Thin films or clouds are observed in the interior, of a slightly reddish tint, floating, as it were, beneath the surface, but which, change their color as the stone is turned so as to receive the light at a different angle. Films, apparently alike, often exhibit different phenomena, some presenting but one color,, some two, while others display a continuous spectrum, and not unfrequently this beautiful play of colors is completely eliminated by polishing. Sulphuric acid causes the opal to turn black, an effect which has led to the opinion that it contains organic matter. The composition of this mineral is not fully assured. Silica
forms its principal constituent, as in quartz, but in a different condition, being amorphous, soluble, and usually hydrous, though water is not considered essential. Its specific gravity and hardness are less than those of quartz, and it is generally supposed to be incapable of crystallization. Opal has been found in the cavities and fissures of volcanic
igneous rocks, porcelain earth, limestone, and the silicious waters of hot springs. It is a natural production of Ceylon, Australia, Hungary, Iceland, the Hebrides, Ireland, Faroe Islands, Honduras, Mexico, and the United States, though here it is not usually in a condition suitable for gems. Pliny, who |
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