Gemstone associated with the month of one's birth. Wearing one was popularly supposed to bring good luck or health. The ancient belief that supernatural powers could be attributed to gemstones is still held by some people today. The list of genuine stones (e.g., diamond, emerald, sapphire, ruby) was supplemented in the 20th century by a series of synthetic stones for use as less costly substitutes.

Learn more about birthstone with a free trial on Britannica.com.