Stone ingots are found on Marble Elementals which can be found by using a gargoyles pickaxe in caves or dungeons.

Each Marble Elemental will provide a small amount of stone ore once killed. Equivilant to 1-4 ingots worth.

To Smelt the ore you need at least 50 mining.

Marble Elementals are found where you find Verite and Valorite ore.