Soulstone can be upgraded once all 6 options have been unlocked. There are 6 Soulstone levels, and each time you upgrade a stone to the next level, the option values increase.
Fuse Soulstone to obtain random Soulstone. Narr Soulstone can only be acquired through this process. Soulstone that can be acquired through fusion changes periodically.
To upgrade your Soulstone, you will need Gold, Soul Powder, and 5 Soulstones of the same grade.