These stones vary much in size; some, particularly those of Siberia, being extremely small, and others being upwards of an inch in thickness. In the Collection of Natural History at Paris there is a Brazilian topaz which weighs four ounces and a quarter. These stones are not sufficiently scarce to be, in general, much valued by the jeweller or lapidary. The deep yellow variety is preferred to the pale sort, although the latter is often superior to it both in size and hardness. Figures have sometimes been engraved on the topaz; and these, when well executed, are of great value. In the National Museum at Paris there is a superb Indian Bacchus engraven on a topaz. The cabinet of the Emperor of Russia contains several fine topazes of this description. Some of the coarse kinds of topaz are broken down, pounded, and used instead of emery for the cutting of hard minerals; and powdered topaz was formerly kept in apothecaries’ shops, and sold as an antidote against madness. It is a somewhat singular circumstance, that, if the Saxon topaz be gradually exposed to a strong heat in a crucible, it will become white and, on the contrary, that Brazilian topazes by the same process become red or pink. By exposure to a still stronger heat, the Brazilian topaz changes its colour to a violet─blue. Jewellers usually divide topazes into the following kinds: 62. BRAZILIAN and SAXON, already mentioned. 63. BOHEMIAN.—These are found chiefly in the tin mines of Bohemia, are of small size, deficient in transparency, have only grey or muddy white colours, and are of little value. 64. BLUE TOPAZ.—This is a large Brazilian gem, which varies in size from one or two carats to two or three ounces. A fine blue topaz, without flaw, and which weighed an ounce and a quarter, was sold for 200 guineas. It is sometimes difficult to distinguish a blue topaz from an aqua marine (68). 65. PINK TOPAZ.—Some beautiful rose─coloured varieties of topaz have been brought from Asia Minor, and others are found in South America; but the pink topazes in the jewellers’ shops are chiefly stones of the yellow Brazilian kind, which have had their colour changed by heat. 66. The WHITE, or NOVA MINA TOPAZ, is a perfectly colourless and transparent variety. It generally occurs of small size, and is in considerable estimation in Brazil for ear─rings, or for being set round yellow topazes. Small stones of this description have recently been found at St. Michael’s Mount, in Cornwall. There is imported from Brazil a yellow kind of crystal (83), which is so similar, in its appearance, to the yellow topaz as sometimes to be imposed upon purchasers for that stone. 67. _The EMERALD is a well─known gem, of pure green colour, and somewhat harder than quartz._ _Its natural form is a short six─sided prism; but it is sometimes found massive, and rounded like a pebble._