Amethysts and Wine
The ancient Greeks loved wine and were always searching for ways to drink without getting drunk. They finally came up with what they thought was the antidote to the downside of Dionysus: drinking purple wine from a purple vessel made of semi-precious stone would cause the two purples to cancel each other out and negate whatever was in the wine that caused drunkeness.
In Greek, the prefix a means "not," methyein means "drunk" (from methy-wine), so the work for "not drunk" became the name of the purple stone the vessel was made out of - amethyst.
Taken from Cuisine and Culture