Boomslang Skin is the shed skin of the Boomslang, a magical snake. The books refer to the ingredient as “shredded Boomslang Skin”, suggesting it is typically prepared by being cut or torn into small pieces for use in Potions. As it is a restricted ingredient, it is not available for general purchase at apothecaries like the one in Diagon Alley and is kept under lock and key at Hogwarts. The Boomslang itself is a venomous snake, which likely contributes to the potency and complexity of potions it is used in.
The primary and sole known use of Boomslang Skin is as a crucial ingredient in the brewing of Polyjuice Potion. It is one of the potion's core components, which also include Lacewing Flies, Fluxweed, Knotgrass, Leeches, and powdered Horn of Bicorn. The recipe, found in the book Moste Potente Potions in the Restricted Section of the Hogwarts Library, requires the Boomslang Skin to be added after the Fluxweed and Knotgrass have brewed. Due to the powerful and dangerous nature of Polyjuice Potion, its ingredients are not easily accessible to students. The presence of Boomslang Skin in Professor Snape's private stores highlights its controlled status within Hogwarts.
Boomslang Skin plays a pivotal role in Harry Potter and the Chamber of Secrets. After deciding to brew Polyjuice Potion to investigate whether Draco Malfoy is the Heir of Slytherin, Hermione Granger discovers that Boomslang Skin is a necessary ingredient that they cannot acquire through normal channels. To obtain it, the trio devises a plan during a Potions class:
This act of theft is critical, as it allows the trio to complete the Polyjuice Potion. The subsequent use of the potion enables Harry and Ron Weasley to transform into Crabbe and Goyle, infiltrate the Slytherin Common Room, and question Malfoy, which proves to be a key step in their investigation into the Chamber of Secrets.