PERIGORD-STONE, an ore of manganese, of a dark grey colour, like the basalt or trapp. It may be scraped with a knife, but is extremely difficult to
be broken. It is found of no regular figure, is very compact, heavy, and as black as charcoal. Its appearance is glittering and striated, like the ore of antimony; its particles being disposed in the form of needles, crossing one another without any agglutination, in such a way that some are loose as iron filings when stuck to a loadstone; resembling the scoria from a blacksmith's furnace. By calcination it becomes harder and of a reddish-brown colour, but is not attracted by the magnet. It has a considerable specific gravity, does not melt per se, but with borax runs into a glass of the colour of an amethyst. It is scarcely affected by nitrous acid without the addition of sugar. It seems also to contain some argil and iron. It is met with in Gascogne and Dauphine in France, and in some parts of England. It is employed by the French potters and enamellers in the glassy varnish of their earthen wares.