Tea bags are made from Abaca, the Philippine word for Manila hemp (Musa Textilis), a type of banana tree. Abaca is a strong fiber capable of standing up to hard use. Tea bags are no thicker than a standard tissue, but try dipping one of those in boiling water! It is also used to make marine grade rope and canvas shoes.
Abaca is the primary fiber I use in my work due to its remarkable strength and ability to absorb pigment.