According to Greek mythology,  Dionysus, the god of fruitfulness and wine,  was angered by a woman named Amethystos.   Amethystos preferred the goddess Diana to Dionysus.   He sent two tigers to eat Amethystos. Diana rescued Amethystos by turning her into clear quartz (not much of a rescue!)  Dionysus was so sorry that he cried into his wine and then spilled it on the clear quartz, staining it purple and creating the first amethyst. 

The name amethyst is from the ancient Greek word "amethustos" which means "not drunk."    Because of the Dionysys myth, the Ancient Greeks thought that drinking from a vessel fashioned from amethyst would keep them from getting drunk.  They probably had a lot of fun testing that theory! 

In Exodus 28-19 in the Bible, the last gemstone in the third row of the High Priest's breastplate is thought to be amethyst by most scholars. 

In Hebrew, the name for amethyst is 'ahlamah' from the word halam meaning 'to dream.'   They thought wearing amethyst would give you good dreams.

In medieval times, amethyst was believed to keep your thoughts pure and make you more intelligent.

In the early Christian church, amethyst was also believed to have spiritual qualities and was part of a bishop's regalia.  The best quality amethyst is often called "bishop's grade."