The fire opal occurs in its greatest perfection in the porphyritic rocks at Zimapan in Mexico. It is generally of a translucent hyacinth-red color and flashes forth dazzling beams of fiery carmine-red with yellow and green reflections. This Mexican gem is the most beautiful and gorgeous of all the varieties of opal; but, alas! it is also the most sensitive, and is frequently irreparably injured by water or exposure, or even by sudden atmospheric changes. So easily affected are the opals by the vicissitudes of the weather that they are almost always brighter in summer than in winter. But there are some varieties that are not so easily influenced, and are not injured by contact with water. The fact that this variety of opal is injured in course of time by contact with moisture or careless exposure is not remarkable when some of the harder gems undergo a change from similar exposure. The hard amethystine quartz, when worn as a finger ornament, will completely bleach out and become colorless in a few years. The black opal is the product of art, and for this purpose harlequin opals are used. The harlequin opal is simply the matrix of other gems spotted here and there with flakes of color dispersed over an opaque ground, and its name was suggested by the resemblance to the motley tints of the harlequin’s dress. Masses of the matrix, with fragments or specks of opal interspersed in its substance, are soaked for a time in a saccharine solution, and afterwards in diluted sulphuric acid. The porous parts of the matrix absorb a minute quantity of the solution, which is afterwards charred by the sulphuric acid; while the solid and transparent parts remain unchanged and exhibit an increased play of colors upon the black ground.