We were chatting about birthstones at the sterling silver pendant workshop last week, and someone asked where the idea of having a stone linked to each month came from. I said I'd find out the answer - and here it is! Actually, my RE teacher would say that I should have remembered, but my only excuse is that I did do my GCSEs nearly 20 years ago!
The traditional birthstones are said to be based on the gemstones used in the breastplate of the first Jewish high priest - Moses' brother Aaron. The breast plate was designed to show the colours associated with the twelve tribes of Israel, and the 12 stones representing those colours were places in 4 rows. Over time, the fact that there were 12 stones became more important as there are also 12 Apostles, 12 months, 12 signs of the zodiac etc, and people began to associate each stone with a particular month.
So there you go! If you want to read more then have a look at these sites: