Rainbow Agate is a name appropriately applied to agates showing a beautiful prismatic effect. These are composed of quartz and chalcedony in very fine layers. The writer secured a splendid specimen of this type of agate set in a jewel which had formed part of an old Saxon collection; it may possibly have come from India. The prismatic play of color differs from that observed in quartz iris, in that the iridescence is due to the minute interference lines and not, as with the iris, to internal fractures. The greatest interest was manifested in the eighteenth century in these agates, one of which was described in a special pamphlet under the title, “Regenbogen Achat,” and illustrated with a colored plate. The effect was that of a spectrum rather than the iris effect of the crystalline quartz. This iris was also highly valued, and great favor was set upon brilliant examples of what was in reality rock-crystal fractured, the small fracture-planes causing the breaking up of the light and producing the rainbow or iris effect. In fact it was a spectrum produced by the mixture of quartz between the chalcedonic layers. Cellini has a marvellous story to tell of a luminous carbuncle. A certain Jacopo Cola, a vine-grower, going into his vineyard one night noticed what appeared to be a bit of glowing coal at the foot of one of the vines, but on reaching the spot he was unable to locate the source of this radiance. Very wisely he retraced his steps to the spot whence he had first observed the light, which became again apparent, and when he now very carefully approached the vine he found that the gleam proceeded from a rough little stone, which he joyfully picked up and carried off with him. He showed it to a number of his friends and among them chanced to be a Venetian envoy, an expert on precious stones, who immediately recognized that the find was a carbuncle. Thereupon taking a base advantage of the finder’s ignorance, he succeeded in buying the stone for only ten scudi, and then hastened away from Rome, lest his deception should be discovered. Not long afterwards this same Venetian went to Constantinople and sold the stone to the Sultan of the time for 100,000 scudi, a profit of 10,000 per cent.[662] The fact that the vintner could only see the gleam from a given spot is in itself sufficient proof that what he noted was merely the reflection of some distant light striking a smooth surface of the stone at a certain angle. Among the many virtues credited to carnelian by the Mohammedans may be noted its power to preserve the equanimity and gravity of the wearer in the midst of disputes or inordinate laughter. A special and peculiar utilization of this material was to employ splinters of it as toothpicks. Their use not only whitened the teeth but also prevented bleeding of the gums. The Prophet, according to tradition, asserted that the wearer of a carnelian ring would never cease to be happy and blessed.[663]