Overview
The Mythic Stones look like black branches.Mythic stones only appear after you have uncursed all the cursed tools(refer to Winter Mine section for more info). Even then, they are very rare and hard to come by. So don't be upset that you cannot find them. Havepatience. It's worth it
Mythic Stones seem to have a higher chance of appearing on levels same as those of the Goddess Jewels(ex. B60, B102, etc). It is just an observation, I do not have any proof to support it. So it all depends on you to believe it or not. Also, it seems that the Stones have a higher chance of appearing at very low levels of the mine. I have managed to find them on levels like B250 and below.