Non-crystalline, massive, stalactitic or nodular; hardness, 6; specific gravity 2; all colors; luster vitreous, resinous, or pearly; transparent on thin edges. Opal differs from chalcedony in having water, usually about 10%, incorporated in its structure. This is water of crystallization, and not firmly held; so that, if opal is heated in a closed tube to above 100 C., it is given off as a vapor. Opal is distinguished from chalcedony by its lesser hardness, and the resinous to pearly luster. It forms in cavities, in layers often of extreme thinness. Opal is originally the product of the dissolution of silicate minerals in hot acid waters, the resulting gelatinous silica, when it is deposited and hardened, becoming the opal. There are many varieties, some of them highly prized as gems in spite of the moderate hardness and opacity of the mineral. Gem-quality opal gets its opalescent character from the successive deposition of thin films of opal, the light penetrating and being reflected from different films. This breaks up the white light and causes the play of colors which is the charm of this gem. Precious opal, in which the play of colors is finest, comes mostly from Hungary, Mexico, and Queensland. The opal was a favorite stone from before Roman times, and in its early history was a charm against the “evil eye.” During the nineteenth century for some reason it came to be considered an unlucky stone. Fire opal is a hyacinth-red to honey-yellow variety, which has a fire-like play of color, and is found in Mexico and Honduras. Common opal does not have the play of color, but comes in a variety of colors; is waxy or greasy in luster; and occurs mostly as fillings of seams or cavities, especially those in igneous rocks, like the steam holes in lavas, etc. It is found in Cornwall, Penn., in Colorado, California, etc. Opal-agate is a variety in which there are color bands, and it is widely distributed. Opalized wood is formed in exactly the same manner as agatized wood, much of the fossil wood called silicified being really opalized. Siliceous sinter is the porous mass of opal which is so frequently deposited about hot springs and geysers. It is readily recognized by its porous character. The shells of the diatoms, which are microscopic plants, are made of opal; and while they are so small, there is certainly no other plant so abundant or omnipresent, living as it does in every pool, lake, or sea by the millions. These shells are very indestructible so that they accumulate at the bottom of ponds, bogs, and sea-bottoms, making at times extensive deposits. This material in quantities is termed diatomaceous earth, or tripolite (from Tripoli where it was first used commercially). It is used as a polishing powder for metals, marble, glasses, etc. The Feldspars