agate as it is called, green jasper, and also green glass. The piece of green glass in the monastery of Reichenau, which is seven inches in length, three inches in thickness, and weighs twenty-eight pounds three-quarters; and the large cup at Genoa, which is however full of flaws[167], have been given out to be emeralds even to the present time. Mirrors were made also of rubies, as we are assured by Pliny[168], who refers to Theophrastus for his authority; but this precious stone is never found now of such a size as to render this use possible; and Gary and the anonymous Italian before-mentioned have proved very properly that Pliny has committed a gross mistake, which has not been observed by Hardouin. Theophrastus, in the passage alluded to[169], does not speak of a ruby, but of the well-known black marble of Chio, though he calls both _carbunculus_, a name given to the ruby on account of its likeness to a burning coal, and to the black marble on account of its likeness to a quenched coal or cinder; and the latter, as well as the obsidian stone, was used sometimes for mirrors. The account how mirrors were formed by the native Americans, before they had the misfortune to become acquainted with the Europeans, is of considerable importance in the history of this art. These people had indeed mirrors which the Europeans could not help admiring. Some of them were made of black, somewhat transparent, vitrified lava, called by the Spaniards _gallinazo_, and which is of the same kind as the obsidian stone employed by the Romans for the like purpose. Of this substance the Americans had plane, concave, and convex mirrors. They had others also made of a mineral called the Inca’s stone[170], which, as has been already said by Bomare, Sage, Wallerius, and other mineralogists, was a compact pyrites or marcasite, susceptible of a fine polish; and on that account often brought to Europe, and worn formerly in rings under the name of the stone of health. Ulloa says the Inca’s stone is brittle, opake, and of a somewhat bluish colour; it has often veins which cannot be polished, and where these veins are it frequently breaks. The mirrors formed of it, which he saw, were from two to three inches in diameter; but he saw one which was a foot and a half. The opinion which some have entertained, that these mirrors were cast, has no other foundation than the likeness of polished marcasite to cast brass. This mineral is very proper for reflecting images; and I am inclined to think that the Peruvians had better mirrors than the Greeks or the Romans, among whom we find no traces of marcasite being employed in that manner. It appears, however, that the Indians had mirrors also of silver, copper, and brass[171].