After passing a particular stage, you will get the chance to collect Mythstones after defeating a Boss. Mythstones are used to craft and level up Artifacts. Once done, they will give you very good bonuses. You will retain them after you go through the prestige process. However, they are very expensive. Always upgrade an Artifact if it costs less than purchasing a new one, but only if you are satisfied with the bonuses it will give you. If not wait to get enough Mythstones to purchase a new Artifact.