Silk can be bought from various silk traders around Runescape. Some of these are located in Varrock, Eastern Keldagrim, Ardougne Market (can only be sold, not bought), and in Al Kharid (3 coins - or 2 - if the player refuses the first price). Silk cannot be made by a player in the game.
They can also be sold to the Silk merchant in East Ardougne for as much as 60 coins a piece after offering to sell for 120 coins first. However, the silk merchant will refuse to deal with the player if they have stolen silk from the stalls within the past 30 minutes of in-game time.
Silk will only be bought from the player if it's un-noted.
Silk is used in a couple of quests such as Dragon Slayer to open the magic door in the dwarven mines and Enlightened Journey to make the Hot Air Balloon. Silk is also used in the Ghosts Ahoy quest to repair the model ship.
This item can also be bought or sold at: