91. _CALCEDONY is a species of quartz, generally of whitish, bluish, or smoke─grey colour; and, when broken, it appears internally dull, and somewhat splintery._ _It is generally found in a massive state, is harder than flint, generally semi─transparent, and 2½ times heavier than water._ The name of this stone is derived from Chalcedon, in Upper Asia, whence it appears to have been originally obtained, and where it is still found in considerable abundance. Several superb specimens of calcedony have been found in Britain, and particularly in some of the tin and copper mines of Cornwall. It occurs in several parts of Scotland; and in many of the countries of the Continent. In the Leverian Museum there was a specimen of calcedony, which weighed more than 200 pounds. Its whole surface appeared such that, at first sight, one might imagine it to have formerly been in a liquid state: it had much the appearance that thick oil has while boiling. Few stones are susceptible of a higher or more beautiful polish than calcedony. Hence the different varieties of it are cut into ring and seal stones, necklaces, ear─pendants, small vases, cups, and snuff─boxes. 92. _ONYX is a kind of calcedony, generally marked alternately with stripes of white and black, or white and brown._ Its name is derived from the Greek language, and has been given on account of its resemblance in colour to the whitish band at the base of the human nail. The distinction which appears to be made betwixt onyx and _sardonyx_, arises from the colours of the former being arranged either concentrically, or in a somewhat confused manner, and those of the latter in regular stripes or bands. Both these kinds are highly esteemed by lapidaries, for the formation of vases, snuff boxes, and trinkets of various kinds. Of sardonyx the ancients made those beautiful cameos, many of which still ornament our cabinets. The ingenuity they have shown, in the accommodation of the natural veins and marks of the stone to the figures engraven upon them, is such as to excite, in many instances, the greatest admiration. It is said that we are entirely ignorant of the country whence the ancient artists obtained the large specimens of sardonyx which are now found in some cabinets. Onyx is imported from the East Indies, Siberia, Germany, and Portugal. 93. _CARNELIAN is another kind of calcedony usually of a red or flesh colour, though sometimes white, orange, or yellow._ On several of the British shores carnelians are found with other pebbles: but the most beautiful and valuable kinds are imported from the East Indies. These are sometimes so large as to measure nearly three inches in diameter. The kinds principally in request are those of pure white, and bright red colour; and jewellers have the art of changing the colour of the yellow varieties to red, by heat.