300	GEMS AND PRECIOUS STONES IN THE
	The rounded side was often curiously carved and decorated. (See Fig. 17.) They also carved pyrite into other objects, notably a human head 2 inches high, in which were inserted eyes of white chalcedony, now in the Blake Collection in the United States Na­tional Museum, and a number of mirrors, now in the Trocadeio Museum, Paris. (See Fig. 17.)
	Mexican onyx, so called, is really an aragonite. Prof. Mariano Barcena, of the Mexican Commission to the World's Fair held in Philadelphia during 1876, has recently published an account of its occurrence and chemical character.' The principal deposits are located near the town of Tecali in the State of Pueblo. It is essentially a carbonate of calcium, containing small quantities of the oxides of iron and manganese, to which are due the varie­gated colors for which the rock is so much admired. The specific gravity, 2 '9, shows that it is aragonite. It was extensively used by the ancient Mexicans, specimens of whose handiwork we still have preserved in our museums in the form of masks, idols, and a vari­ety of objects. The softness of the material (it can be readily carved with a knife) has tempted some of the modern residents of Mex- 1 The Rocks Known as Mexican Onyx, Proc. Acad. Nat. Sci., Phila., Vol. 28, p. 166, 1876.