Although it looks like heavy weight paper, the charter is written on parchment.
Parchment is made from the untanned skins of animals, most usually sheep, calves and goats. Vellum is a special kind of parchment only made from calf skin or occasionally other young animals. Making parchment is a long process. First, the skin is removed from the animal and all the hair and flesh is cleaned away. Then the skin is stretched on a wooden frame. The surface of the skin is then scraped with a special curved knife called a lunellum. The skin is then wetted and dried before another round of scraping, wetting and drying. This continues until the skin is the right thickness and tautness for use. Sometimes the skin will then be rubbed with a pumice stone and chalk to enable the surface to better accept ink.
Parchment has traditionally been used for important documents because it is considered more stable than paper. This means it is less susceptible to deterioration and will last much longer. The US Constitution and Declaration of Independence are written on parchment.