They are sometimes set in rings; and the prices at which they were formerly valued were, in the highest degree, unaccountable and absurd. At present their value is considerably lower, though they are still in great request as objects of curiosity. The phenomenon of their becoming transparent in water is supposed to be occasioned by that fluid soaking through their whole substance, in the same manner as the transparency of paper is occasioned by immersing it in oil. An hydrophanous opal weighing 27½ grains was kept four minutes in water, and, on being taken out, weighed 32½ grains, having received in this short period an augmentation of five grains, or more than one sixth part of its whole weight. When taken from the water, these stones as they dry become again opaque. To preserve them in beauty and perfection, care should be taken not to immerse them in any but pure water, and to take them out as soon as they have acquired their full transparency. If these precautions be neglected, the pores will soon become filled with earthy particles: the stones will cease to exhibit their peculiar property, and will ever afterwards continue opaque. 104. COMMON OPAL _is a semi─transparent kind of opal, which does not exhibit any changeable refraction of colour_. It is found in Germany, France, Italy, and other countries of the Continent, and is employed for brooches and other ornaments. A green─coloured Saxon variety is sometimes cut into ring─stones. 105. MOTHER─OF─PEARL OPAL, _or_ CACHOLONG, _is a milk─white, yellowish, or greyish─white kind of opal_, which occurs in Iceland, Greenland, Spain, and the island of Elba. It is sometimes cut into a concave form, for brooches, and other female ornaments. Italian artists also use it for mosaic work. 106. WOOD OPAL _appears to be wood that, by some extraordinary operation of nature, has been converted into opal_. Some specimens exhibit, very beautifully, the ligneous texture. This kind of opal is chiefly cut into plates for the tops and bottoms of snuff─boxes. It is found in alluvial land in some parts of Germany and Hungary. Several years ago the trunk of a tree, penetrated with opal, and so heavy that eight oxen were requisite to draw it, was found in Hungary. PITCHSTONE FAMILY. 107. _OBSIDIAN is a kind of glass, generally of blackish colour, formed in volcanoes, from which it issues in thick streams._