The Earth Rune is one of the least popular elemental runes. While found on numerous characters, these runes are often swapped for the more powerful Mother Earth Rune or simply for another element. Suikoden III was the first game to really spotlight the earth element by tying it to the Grassland tribes through Aila and Yumi. Rather than merely carrying the Earth Rune, they were both greatly tied to the spirits of the earth itself.
The earth element provides mainly defensive and support magic as well as some attack spells which mostly target enemies on ground only. It can be used to raise the defensive power of your party and creates full magical protection for the upcomming spell from your enemies.
The Earth Rune is sold in many shops, being a normal elemental magic rune. Your first chance to see one is at the right hand of Eileen after you saved her in Kouan. After that, more crystals happen to exist at the stocks of Rikons salesman. Surprisingly there are few enemies of which this rune can be looted from. - Arya, Celes