Seaweed

Seaweed itself doesn't have very much use. (It's not to be confused with its relative, Edible seaweed.) However, when used on a range, it can be burned down to make soda ash, which is a key element in making molten glass.

Seaweed can be found all over RuneScape along the shoreline, especially in Karamja and Entrana. It is also caught often in a big net while fishing, as well as being sold by Traders.