The seal is made of silicone. Silicones are rubber like materials that are primarily made of silicon and oxygen. They are frequently used in the manufacture of kitchen implements including those used for baking because of their non-toxicity and excellent durability including resistance to heat. The silicone is made in the USA and are FDA and EU approved for use in contact with food.
Note on older seals: Up until October of 2018 the AeroPress seal was made of a thermoplastic elastomer. Both the silicone and the thermoplastic elastomer are made in the USA and are FDA and EU approved for use in contact with food.