Check out your quest journal when in doubt. The mine is on the Northern side of Ikko island. There's only one big pink crystal there and the boss will block the entrance to it. You don't need to solve the puzzle to get it, just beat the boss.
thanks, I have another question, in MonGirl Sexpedition I'm at the step that meet friends at my home, but I realized even after I return to the home cave, they are not showing up. btw your game are the best :)
If the quest journal says so, they should be there during day time. If it's not the case, send me your entire save folder zipped and I'll check it out.
