The birthstone for October is morganite, though the most common or modern stones are the opal and tourmaline. Tourmaline, is also commonly referred to as the "pink emerald". Opals and tourmaline are birthstones that are also associated with the month of October, and morganite is the rarest of the three. Morganite is available in almost every color of the rainbow. Unfortunately, this type of stone sends to be prone to cracks, flaws, and inclusions. Therefore, in terms of gifts, of the three, opal is the most common birthstone. During the Renaissance Period of Europe, the opal was considered to be a "mood ring" or sorts, indicating the mood of the wearer. However, in the 14th century, the stone was thought to be responsible for the development of the Black Plague. Also available in a wide range of colors, the black opal is considered to be the most valuable.