Seaweed

Seaweed itself does not have very much use. However, when used on a range, it can be burnt 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 on southern Karamja and north-western Entrana.

It can NO LONGER be bought from Arhein in Catherby for 2 coins each (or any other trader that I've found)!

This only leaves finding it or catching it in a big net while fishing as the only means to acquire it.

The Superglass Make lunar spell takes either seaweed or soda ash to make molten glass so if the spell is used, there is no need to burn your seaweed into soda ash.