which being made into millstones, by virtue thereof flour is made; wherefore ’tis a common saying in those countries that blessings come from God and millstones from Montisci:  but, for that these sandstones are in great plenty, they are held cheap by us, just as by them are emeralds, whereof they have mountains, bigger than Monte Morello, that shine at midnight, a God’s name!  And know this, that whoso should make a goodly pair of millstones, and connect them with a ring before ever a hole was drilled in them, and take them to the Soldan, should get all he would have thereby.  The other sort of stone is the heliotrope, as we lapidaries call it, a stone of very great virtue, inasmuch as whoso carries it on his person is seen, so long as he keep it, by never another soul, where he is not.”  “These be virtues great indeed,” quoth Calandrino; “but where is this second stone to be found?” Whereto Maso made answer that there were usually some to be found in the Mugnone.  “And what are its size and colour?” quoth Calandrino.  “The size varies,” replied Maso, “for some are bigger and some smaller than others; but all are of the same colour, being nearly black.”  All these matters duly marked and fixed in his memory, Calandrino made as if he had other things to attend to, and took his leave of Maso with the intention of going in quest of the stone, but not until he had let his especial friends, Bruno and Buffalmacco, know of his project.  So, that no time might be lost, but, postponing everything else, they might begin the quest at once, he set about looking for them, and spent the whole morning in the search.  At length, when ’twas already past none, he called to mind that they would be at work in the Faentine women’s convent, and though ’twas excessively hot, he let nothing stand in his way, but at a pace that was more like a run than a walk, hied him thither; and so soon as he had made them ware of his presence, thus he spoke:—­“Comrades, so you are but minded to hearken to me, ’tis in our power to become the richest men in Florence; for I am informed by one that may be trusted that there is a kind of stone in the Mugnone which renders whoso carries it invisible to every other soul in the world.  Wherefore, methinks, we were wise to let none have the start of us, but go search for this stone without any delay.  We shall find it without a doubt, for I know what ’tis like, and when we have found it, we have but to put it in the purse, and get us to the moneychangers, whose counters, as you know, are always laden with groats and florins, and help ourselves to as many as we have a mind to.  No one will see us, and so, hey presto! we shall be rich folk in the twinkling of an eye, and have no more need to go besmearing the walls all day long like so many snails.”  Whereat Bruno and Buffalmacco began only to laugh, and exchanging glances, made as if they marvelled exceedingly, and expressed approval of Calandrino’s project.  Then Buffalmacco asked, what might be the name of the stone.  Calandrino,