I only found this myth today, and have to share it with you.
Bacchus, the God of Wine and Intoxication, had fallen under the spell of Diana, Goddess of the Hunt.
Bacchus was certainly trying his luck here, as Diana is a virgin Goddess, holding sacred her chastity.
Bacchus made his advances towards the beautiful Goddess, and was swiftly rebuked.
This made the God furious, so he turned to his beloved wine, quickly reaching a rolling state of drunkenness.
The drunker he became the more his anger boiled.
Drunk and plotting revenge, Bacchus swore that the next mortal maiden who crossed his path, he would put to death.
A sweet young maiden, by the name of Amethyst was making her way to the altar of Diana.
Bacchus marked her for his revenge and set loose a pair of ravening tigers to swiftly dispatch the girl.
As the tigers approached her, Amethyst asked her Goddess for protection against the maddened beasts.
Diana is the Goddess of the Hunt, but even she had no time to draw her bow.
Thinking quickly, she transformed Amethyst into a crystal, bright, clear and sparkling.
Upon Amethyst’s transformation, Bacchus had a moment of clarity, realising that the mortal girl had nothing to do with his grudge against Diana.
Full of remorse, he got on his knees before the crystal and begged for Amethyst’s forgiveness.
He then took his sacred wine and poured a libation over the sparkling gem.
The wine soaked the crystal, turning it a beautiful shade of purple.
Bacchus had previously imbibed most of the wine, and the libation he poured did not soak the crystal completely.
This is why Amethyst is often clear at the base and purple on top.
It is said that Amethyst prevents drunkenness, as the wine is already contained within the crystal.