Metamorphoses Book 2: The Envy of Aglauros
While Mercury was flying over Athens after hiding Apollo's herd, he saw a beautiful girl taking part in the festival of Pallas Athene. She was one of the daughters of Cecrops, Herse. Mercury entered Cecrops' palace and Aglauros, the faithless sister who had broken her vow to Athene, questioned his identity and his intent. When he told her that he was in love with her sister, she agreed to help him win Herse, but only if he paid her a large sum of gold. Because the god did not have the gold with him, she chased him from the palace. Athene saw all that was happening and decided to get her revenge against Aglauros for her betrayal. Athene went to foul Envy's rank hovel and told her to infect Aglauros with her poison and Envy obeyed.
When Mercury came the next day to see Herse, Aglauros blocked the doorway and would not let him in. She was jealous of the god's affection for her sister, and she swore that she would not move until he was forced away. After she uttered those words, Mercury transformed her into a statue, and her dark and evil thoughts stained the statue's stone.