Juju knives are items that is required to perform most spells. Knives vary greatly in shape, size, color and material, as shown when Sunny goes to buy hers and sees several, made from materials like metal, wood and even plastic.
Juju knives choose their owners, and when a person has the right juju knife for them, they can feel through the blade as if it's part of their body. This happens when Sunny first wields her knife in Akata Witch. (page 220)