Ground guam is made by using a pestle and mortar on a guam leaf. It is used to make fish food by combining it with ground seaweed on an empty box.
Ground guam cannot be used to make potions, such as attack potions.
With a pestle and mortar in your inventory and a Herblore level of 19, you can use a ground guam on a stack of (at least 15) swamp tar to make 15 guam tar, granting 30 Herblore experience.