Aaron became a high priest. His ceremonial breastplate held four rows of three stones each. Exodus 28:17-20 states, "There were twelve stones, one for each of the names of the sons of Israel, each engraved like a seal with the name of one of the twelve tribes." These 12 stones also symbolized the 12 months of the year and the 12 signs of the zodiac.
Biblical scholars have a difficult time translating exactly what these stones are. The King James Bible lists the stones as: (Row 1) sardius, topaz, carbuncle; (Row 2) emerald, sapphire, diamond; (Row 3) ligure, agate, amethyst; (Row 4) beryl, onyx, jasper. The New American Standard Bible lists them as: (Row 1) ruby, topaz, emerald; (Row 2) turquoise, sapphire, diamond; (Row 3) jacinth, agate, amethyst; (Row 4) beryl, onyx, jasper.
The gems have changed a few times and different countries use different stones. Below is the US version for 2012.
It was in 15th-century Poland that wearing these birthstones gained popularity. In contrast to today's custom of wearing your birthstone throughout the year, the early proponents owned a full set of 12 and wore each month's stone, regardless of birthday. The Gemological Institute of America says the custom began in Germany in the 1560s.