Birthstones are gemstones that are associated with each month in the Gregorian Calendar. They may be precious stones - those rare and expensive gems such as diamonds, emeralds, rubies, pearls, and sapphires - or semi-precious, which includes practically all the others such as ameythst, opal and bloodstone.
In each case they are considered lucky for their particular months and their healing powers are allegedly heightened during their months. Thus, if you are born in January, then your birthstone is the garnet, and you will get the most benefit from this stone by wearing it in January.
However, as each birthstone has heightened powers during its month, some people will buy a set of all twelve and rotate them for each month.
The first association of a special gem with each month was recorded in the Bible. Exodus 28 and 39 describe how, in 1250 BC, Moses made a breastplate for the High Priest of the Hebrews according to instructions he received during 40 days in the mountains. The breastplate featured twelve gems which were later linked with the signs of the zodiac, and later still associated with the months in the year.
The breastplate is described as being square with four rows of precious stones: in the first row there was a ruby, a topaz and a beryl; in the second row a turquoise, a sapphire and an emerald; in the third row a jacinth, an agate and an amethyst and in the fourth row a chrysolite, an onyx and a jasper. Mounted in gold filigree settings, each of the were twelve stones - one for each of the names of the sons of Israel - was engraved like a seal with the name of one of the twelve tribes.