A well-marked characteristic of all Topaz crystals is their tendency to cleave across the prism parallel with its base. Such a cleavage plane can be seen cutting across the crystal shown in the upper right-hand corner of the accompanying plate. This cleavage is so marked and the cleavage plane so bright and flat that in cutting Topaz for a gem a cleavage surface is used as the upper face of the gem and the other faces formed around it. Owing to this easy cleavage the owner of a cut Topaz should be careful not to let the stone drop, as it might be cracked or broken. [Illustration: TOPAZ.] Left column: Topaz with Mica and Feldspar (Russia.) Topaz (Brazil.) Center: Topaz in Rhyolite (Utah.) Right column: Topaz (Japan.) Waterworn Topaz (Brazil.)—Loaned by Foote Mineral Co. The name Topaz is derived from the Greek name topazios, which is that of an island in the Red Sea. The gem known to the ancients as topaz, however, was not our Topaz, but the mineral chrysolite. Topaz usually occurs in gneiss or granite, with tourmaline, mica, beryl, etc. In Brazil it occurs in a talcose rock or in mica slate. It is sometimes in sufficient abundance to form an essential rock constituent. When so occurring, however, it has not the transparent gem quality, but is white and opaque. Much of the Brazilian Topaz occurs as rolled pebbles, one of which is shown in the accompanying plate. These occur in the beds of streams, having been left behind owing to their superior hardness after the rock in which they were formed has been washed away. When colorless they are known in the region as “pingos d’Agua” (drops of water). The Portuguese call them “slaves’ diamonds.” A stone in the crown of Portugal, reputed to be a diamond of 1,680 carats weight and called the Braganza, is undoubtedly only a Topaz of exceptional clearness and beauty. The Brazilian Topazes come mostly from the Province of Minas Geraes, the province which also yields diamonds, beryls and many other precious stones. While those of greenish and bluish shades are found mostly in the form of rolled pebbles the yellow Brazilian Topaz is found in the mother rock. This is a decomposed itacolumite of a white or yellow color. The Russian Topazes, like that shown in the plate, come from the Imperial mines in the Urals. Alabashka, near Mursinka, is one of the most productive localities. The crystals occur in cavities in granite and are accompanied by crystals of smoky quartz, feldspar and mica. Superb gems are cut from these Topazes, a fine series of which is possessed by the Field Columbian Museum. The mines are operated by the Russian Government and the finest specimens are reserved for the Imperial Cabinet.