Weak Earth Runes need a Earthstone along with a Blank Rune and a Silver Ingot to make one Weak Earth Rune.
Weak Runes can be crafted at level one at the Rune Altar.
You need three of these to learn Stone Blast and you will need extra to cast the spell.
The command to make a Weak Earth Rune is [nya!rpg rune altar weak earth]. For more info use [nya!info weak earth rune].