until the close of the middle ages it was believed (following Aristotle) that pearls resulted when oysters drank in the rosy glow of dawn, on the surface of the seas. It is indeed a fact that in the early hours oysters rise from their places on the sea bed: they look as if they were taking their bearings.

—We know today that mother-of-pearl consists of tiny crystalline platelets of calcium carbonate, in layers about a hundredth of a millimeter thick. Because the thickness of the layers is approximately equal to the wavelength of the light they reflect, interference occurs, giving the pearls their iridescent appearance.
—The thinner the layer, the more valuable?
—That is why they cannot be artificially constituted. They are “unimprovable.”
—So what could be done to prevent pearl necklaces from bringing so much misfortune?
—My dear fellow, you always think only about raising the stakes.
—What else should I think about?
—Perhaps, for example, how is it that the frequencies of these crystalline platelets and of the dawn light coincide? After all, that increases the desirability of pearls and exposes the oysters to risk.
—Are you suggesting that these subtle creatures have added a further evolutionary development, so that pearls around the throat of a beautiful woman or set in the turban of an Indian prince invariably bring misfortune—that oysters thus ruin the market in pearls? You’re suggesting that this is the oysters’ stratagem for self-preservation?
—In fact the real danger is from the slicks left by oil tankers.
—Has an oyster ever seen a tanker?
—They are highly intelligent, and do possibly possess magical senses, but not exactly eyes. Not eyes; but tears.
—Exactly. The pearls. The oysters experience pain, Aristotle maintains, when pearls are born. They are tokens of injury.