In the 17th century, eminent scholars such as Andrea Bacci, an Italian scientist known for his treatises on the therapeutic quality of water, believed that pearls derived their ability to grow from the lunar phases. Those forming in the waning moon were small while an increasing moon allowed them to become big and beautiful. The atmospheric conditions were also supposed to interfere with their aspect, a cloudy and stormy sky giving dark and opaque pearls. It is known today that the type of oyster and the length of the growth period are responsible for the pearls' color and size.