The Danco 9D00088569 Faucet Washer is made of rubber. However, the specific type or formulation of rubber used in this item is not provided in the product description. It's likely a general-purpose rubber suitable for use in faucet applications. Rubber washers are commonly used in plumbing fixtures like faucets to create a watertight seal between components and prevent leaks. They are designed to withstand water exposure and provide durability over time. If you require information about the specific type of rubber used in the Danco 9D00088569 Faucet Washer, I recommend contacting Danco directly or referring to the product documentation for more details. They should be able to provide you with additional information about the material composition of the washer.