Just as we considered opal with quartz (because of its chemical similarity) when discussing mineral species, so we may now consider the proper naming of opals here. "_Precious opal_" is distinguished from "_common opal_" by the beauty of its display rather than by any difference in composition. The effect is of course due to the existence of thin films (probably of material of slightly different density), filling what once were cracks in the mass. The rainbow colors are the result of interference of light (see a college text on physics for an explanation of interference). The varying thickness of these films gives varying colors, so different specimens of opal show very different effects. The differences of distribution of the films within the material also cause variations in the effects. Hence we have hardly any two specimens of opal that are alike. There are, however, certain fairly definite types of opal and jewelers should learn to apply correct names to these types. Most prominent among the opals of to-day are the so-called "_Black opals_" from New South Wales. These give vivid flashes of color out of seeming darkness. In some positions the stones, as the name implies, appear blue-black or blackish gray. By transmitted light, however, the bluish stones appear yellow. Owing to the sharp contrast between the dark background and the flashing spectrum colors, black opals are most attractive stones and fine specimens command high prices. One fine piece, which was on exhibition at the Panama-Pacific Exposition was in the shape of an elongated shield, about 1-3/4 inches by 1-1/8 inches in size and rather flat and thin for its spread. It gave in one position a solid surface of almost pure ruby red which changed to green on tipping the stone to the opposite direction; $2,000 was asked for the piece. "_White opal_" is the name applied to the lighter shades of opal which do not show the bluish-black effect in any position. "_Harlequin opal_" has rather large areas of definite colors giving somewhat the effect of a map of the United States in which the different States are in different colors. "_Fire opal_" is an orange-red variety. It has some "play" of colors in addition to its orange-red body color. "_Opal Matrix_" has tiny specks and films of precious opal distributed through a dark volcanic rock and the mass is shaped and polished as a whole. JADE. "_Jade_" should next receive attention. It is a much abused term. Under it one may purchase _jadeite_, _nephrite_, _bowenite_, _amazonite_, or frequently simply _green glass_. The use of the word ought to be confined to the first two minerals mentioned, namely, jadeite and nephrite, for they only possess the extreme toughness together with considerable hardness that we expect of jade. Bowenite, while tough, is relatively soft and amazonite is brittle and also easily cleavable, while glass is both soft and brittle.