Opal
4 of the 7 encyclopedias on this shelf carry an entry for Opal. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a well known mineral. It is composed chiefly of silica. It differs from quartz in that it contains from three to ten per cent more water. It varies greatly in appearance and quality. The lustre may be glassy, pearly, or resinous. In color it may be white, red, yellow, green, gray, or blue. The best effects are obtained from a rounding surface, so the opal is left uncut. The precious opal is obtained chiefly from Hungary and Australia. It had a reputation among the ancients for magical virtue. Wrapped in a bay leaf, it was supposed to render its possessor invisible and to make him irresistible to the object of his affections. The fire opal of hyacinth red is found in Mexico. It was a favorite gem of the Aztecs. A bluish opal is also found in Mexico and Central America . A more common opal, lacking somewhat in beauty, is found in Pennsylvania, Colorado, and California. A forest of fossilized wood having the appearance of opal was found in Montana in 1903. The most noted opal gem is possessed by the crown of Austria. It is guarded with the royal jewels at Vienna. It is a flashing Hungarian stone two and one-half inches in diameter. See Quartz
Collier's New Encyclopedia (1921)
a precious stone of various colors, which comes under the class of pellucid gems. It consists of silica with about 10 per cent. of water, and is very brittle. It is characterized by its iridescent reflection of light. It is found in many parts of Europe, especially in Hungary, in the East Indies, etc. The substance in which it is generally found is a ferruginous sandstone. There are many varieties or species, the chief of which are (a) precious or noble opal, which exhibits brilliant and changeable reflections of green, blue, yellow, and red; (b) fire opal, which simply affords a red reflection; (c) common opal, whose colors are white, green, yellow, and red, but without the play of colors; (d) semiopal, the varieties of which are more opaque than common opal; (e) hydrophane, which assumes a transparency only when thrown into water; (f) hyalite, which occurs in small globular and botryoidal forms, with a vitreous luster; (g) menilite, which occurs in irregular or reniform masses, and is opaque or slightly translucent.
The Encyclopedia of Founding (1892)
A species of the quartz family of minerals, from which it differs in containing 5 to 13 per cent of water. It is a precious stone, consisting principally of silica with some alumina. When first dug from the earth it is soft, but it hardens and diminishes in bulk by exposure to the air. The specific gravity varies from 1.9 to 2.5. Subspecies of this gem are called the semi and the wood opal. It is translucent, usually blue or yellowish white, and exhibits a beautiful play of brilliant colors owing to minute fissures which refract the light. Found in Hungary, Queenstown, and the United States. See MINERALS; PRECIOUS STONES.
An Encyclopaedia of Occultism (1920)
Recreates the heart, preserves from contagion in the air, and dispels sadness; it is also good for weak eyes. Pliny's description of this stone glows with enthusiasm, and he gives the preference to those which are shadowed as it were with the colour of wine. The name poederos, applied to the opal, is understood to indicate the beautiful complexion of youth.