Dragon Fruit - restores MP, HP and conditions. Whether this is correct or not, I do not know. I have a character that is level 99 with almost all of the items, and I can fit at least 5 to a card. So you decide. 2 crystals can be taken to the crystal crafter and then turned into a flask.
Flasks are very important for trading and carrying waters.
You are suppose to be shipwrecked on an unfamiliar island and that is why certain items are not explained in the manual. It's all part of the game to find out what things do. Two ways of doing this are, first find the fortune teller, and she'll tell what items are used for. The Second is the Truth Glass. If you open a door with a Rhombus key, make sure you have another one before you go into that room.