Silver hair net
The hair net looks like a silvery spiderweb. Its fine-spun silver wire strands are so thin and delicate, that the net seems to weigh no more than a breath of air. Small gems are set where any two strands cross. The gems, so dark they drink the moonlight, are supposedly rare black amethysts from Asshai, which are a deep true purple in daylight. However, the "amethysts" are in fact the strangler, a choking poison in the form of deep purple crystals.
A Clash of Kings
After the award ceremony for the Battle of the Blackwater, Ser Dontos Hollard meets Sansa Stark in the godswood of the Red Keep. Dontos gives Sansa a silver hair net, set with gems he claims are black amethysts from Asshai. He says that the hair net is magic, that it will give Sansa justice, vengeance for King Joffrey Baratheon's execution of her father, Lord Eddard Stark, and will take her home to Winterfell.
A Storm of Swords
In the riverlands, Arya Stark and the brotherhood without banners camp overnight at High Heart. They meet with the ghost of High Heart to hear her tell the future. The old woman's dreams says she dreamed of a maid at a feast with purple serpents in her hair, venom dripping from their fangs.
As instructed, Sansa wears the silver hair net for the royal wedding feast, with a matching gown of silvery satin and purple felt. Tyrion Lannister, Sansa's husband, notices that her handmaid, Tyrion's secret lover Shae, has arranged the girl's hair artfully within the silver and purple gemstone net. When they arrive at the feast, Lady Olenna Tyrell says that the wind has disturbed Sansa's hair, and she tucks the loose strands back into place and straightens the hair net.
During the wedding feast, King Joffrey begins choking while eating pigeon pie and drinking wine from his wedding chalice. Bystanders and the Kingsguard are unable to help, and the boy king dies, unable to breathe. Queen Regent Cersei Lannister has her brother Tyrion arrested for the crime of kingslaying.
In the chaos of Joffrey's death, Sansa flees to the godswood as she had been instructed by Dontos, and changes into the darker, simpler clothing she had previously hidden there. When she removes her hair net, she notices that one of the black amethysts is missing, with a dark smudge in the silver socket where the stone had fallen out. Terrified, Sansa realizes that the "magic" of the hair net must be related to Joffrey's murder, that the amethysts must not truly be amethysts. When she hears Dontos arriving in the godswood, she hides the hair net in the pocket of her cloak. Sansa confronts Dontos, saying he must have taken the amethyst from her hair net and poisoned Joffrey, but he denies it. They then escape the Red Keep to a ship in Blackwater Bay, where Dontos is killed by order of Lord Petyr Baelish.
At the tower of House Baelish in the Fingers, Sansa confronts Petyr about Joffrey's death, accusing him of commanding Dontos to poison the king. Littlefinger laughs, and says that the alcoholic Dontos's role was to make sure Sansa wore the silver hair net to the wedding feast and help her escape the castle. Petyr says that someone must have straightened her hair net that night, and Sansa realizes it must have been Lady Olenna who took the "amethyst" from the net and used it to poison Joffrey during the celebration.
—thoughts of Sansa Stark
Sansa: You said I must wear the hair net. The silver net with ... what sort of stones are those?
Dontos: Amethysts. Black amethysts from Asshai, my lady.
Sansa: They're no amethysts. Are they? Are they? You lied.
Dontos: Black amethysts. There was magic in them.
Sansa: There was murder in them!
Dontos: Softly, my lady, softly. No murder. He choked on his pigeon pie. Oh, tasty tasty pie. Silver and stones, that's all it was, silver and stone and magic.