I accidentally used a mana potion on a soulstone when I had no inventory space and it was used up. I checked the potion but the potion had no side affects. Afterwards I tested it with a Gmana potion and it said Nothing happens.
It appears as though mana potions are not suppose to do this and it is just a bug because it just deletes the soulstone. I tested it again once I had another mana potion and a soulstone to spare.

Here is a gyazo of what happens.
https://gyazo.com/4007032f83a833a03f5ec88f43968846

Curious.

Technically it was a bug with the code for mana pots and heal pots. The bug is now fixed.

However, I discovered that it was supposed to do something.
Soul stones have special actions when used with:
- red or blue flowers
- torches
- empty flasks
- heal or mana pots
- plus a stack of armour and weapons which are better known