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, the fool Ser Dontos Hollard meets Sansa Stark in the Red Keep's godswood. Under the secret orders of Petyr Baelish, 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 will give Sansa justice, vengeance for King Joffrey Baratheon's execution of her father Eddard Stark, and take her home to Winterfell.
A Storm of Swords
In the Riverlands, Arya Stark and the brotherhood without banners camp overnight at High Heart to meet with the ghost of High Heart to hear her tell the future. The old woman relates several of her dreams, including one 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 wedding feast, with a matching gown of silvery satin and purple felt. Tyrion Lannister, Sansa's husband, notices that her handmaid Shae has arranged her hair artfully within the silver and purple gemstone net. But when they arrive at the feast, Lady Olenna Tyrell says that the wind has disturbed Sansa's hair, and tucks the loose strands back into place and straightens the hair net.
During the wedding feast, King Joffrey appears to choke on a piece of his wedding pie, and dies, unable to breathe. Tyrion is arrested for the crime of kingslaying. In the chaos of the assassination, 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 Petyr Baelish's ship in Blackwater Bay, where Dontos is killed on Petyr's orders.
At the House Baelish tower in the Fingers, Sansa confronts Petyr about Joffrey's death, accusing him of commanding Dontos to poison him. Littlefinger laughs, and says that the alcoholic Dontos's only job was to help Sansa escape the castle, and make sure she wore the silver hair net to the wedding feast. 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's wine.
—the ghost of High Heart
—thoughts of Sansa Stark
Silver and stones, that's all it was, silver and stone and magic.