Beatrice Stone Locations:
*NOTE: As the other guides say, you need 3 Stones for the achievement to unlock. Once you obtain 3 Beatrice Stones, the other fountains will only give you souls, with a bit of health and mana mixed in there as well.*
1. Limbo - You come to a crossroads of sorts, with the option to push Charon's head or open the demon door. Push the giant head (for an achievement) to find a Beatrice Stone and relic.
2. Lust - When you come to the bridge, head over to the charged plate and go to the right. There's a hidden ledge that lets you drop down. Follow the path and you'll find the Beatrice Stone.
3. Gluttony - In the maze, it is off on the right hand side. To get there, go through the maze normally, but when you get to the second moving platform that would take you out of the maze, double jump to the right when the platform is at the lowest point. Collect your Beatrice Stone, then fall through the door closer to the screen to land on the platform again and exit the maze.
4. Greed - After dropping off a corpse ladder, you'll come to a large ledge. Climb up, then look to the ledge on the right and jump up there. *Don't miss the coin and relic found in these ledges* Do a tough double jump to the top left of the screen, toward the archway. You could also try this from the left, but I found it easier going cross-screen. There is an additional hidden ledge there that has a Beatrice Stone.
5. Anger - After the fight with the fire creatures and temptresses and absolving the shade, the next room has a rope that hangs right to left. Before dropping off the rope and onto the sinking platform below, continue all the way to the left for a mana fountain and a Beatrice Stone.
6. Heresy - You'll come to a second hallway of fire, with Virgil at the end beyond the demon door. Above Virgil's head, there's a corpse ladder to grab onto and climb up to a ledge. There are two fountains, one containing a Beatrice Stone, and a relic up here, so make sure you go around the entire platform.
If you have not gotten 3 by the time you pass Heresy, then you will need to collect them in your next playthrough.