"Amongst the hard stones called vitrifiable... few... strictly possess all the qualities... mentioned; because in very few... the vitrifiable earth is pure. Most of these stones, as hard pebbles of all kinds, sand, free-stone, s, , rock-crystal, and the stones called precious, are deficient in some... qualities required to constitute the purest vitrifiable earth. Some... are opake, or only semitransparent; others... colored; some... fusible by a great heat; and, lastly, others, although much harder than any other kind of stones, want the last degree of hardness; all which prove that they are mixed with heterogeneous substances, chiefly phlogistic, metallic, or even earthy, of a different kind."