The yakut is six-fold: 1, the red; 2, the yellow; 3, the black; 4, the white; 5, the green or peacock-hued; 6, the blue or smoky-hued. Some divide the yakut into four classes: red, yellow, dark, and white, reckoning the peacock-hued and the blue among the dark. The yakut cuts all stones except carnelian and diamond. Although the Oriental carnelian is hard and difficult to cut or polish only popular prejudice accounts for this statement, as it falls far short of the diamond in hardness. Pseudo-Aristotle, writing some time from the seventh to the ninth century A.D., was the first to define clearly the three leading varieties of the corundum gems (yakut) as the same mineral substance, and differing only in color. These are the ruby, the Oriental topaz (jacinthus citrinus) and the sapphire. Instead of according different medicinal or talismanic virtues to these three precious stones, this writer states that each and all of them, when set in rings or worn suspended from the neck, protected the wearer from danger in epidemics, gave him the honor and good will of his fellow-men, and also the privilege of having his petitions accorded.[694] The great Athenian comic poet, Aristophanes (c. 448–c. 385), makes Strepsiades, one of his characters in the “Clouds,” assert to Socrates that he knows of a stone having the virtue of saving him from the payment of a claim of five talents, for which suit has been brought against him. This stone, called ὓαλος in Greek, was to be found in the stock of those who dealt in medicines; it was transparent and with it fire could be kindled. The philosopher, although he knows the stone well enough, fails to see how it could be made to help the defendant in a suit at law, and asks Streposiades what he proposes to do with it. The latter is not at a loss for an answer and declares that when the clerk proceeds to write down the charge on his waxen tablet, he, Streposiades, will hold the stone in the sun’s rays so that its beam of light will fall upon the tablet and melt the wax, thus quite literally “wiping out the charge.”[695] Rock-crystal was so highly prized in Roman times that one of the greatest treasures preserved in the Capitol was a mass of this stone, weighing fifty pounds, that had been dedicated by Livia, wife of Augustus Cæsar. Vessels of great size were also made from this material, one of the largest being a bowl owned by Lucius Verus, the colleague of Marcus Aurelius, the dimensions of which were so great that the stoutest toper of the time could not empty it at a single draught. If we can trust a statement of Mohammed Ben Mansur, the Arabs and Persians of a later age must have far surpassed the Romans in the size of their crystal vessels, for he says that a Mauritanian merchant owned a basin of rock-crystal within which four men could seat themselves at the same time. It is true that this basin was composed of two pieces of the material.[696]