Earth Runes need a Earthstone along with a Blank Rune; a Silver Ingot and three Weak Earth Runes to make one Earth Rune.
Runes can be crafted at Arcane Forging level ten at the Rune Altar.
You need three of these to learn Stone Slash and you will need extra to cast the spell.
The command to make an Earth Rune is [@Neko Chan#7837 rpg rune altar earth]. For more info use [@Neko Chan#7837 item info earth rune].