I cannot find the second diamond key.I found
only diamond gem but this not fit to lock.
Please help

Glowstones?
The mermaid you meet by the grate i the floor will give you one when you talk to her.
The second is in a chest down the halls behind her in a big room with two chests in it (one on the floor & one in the rafters).
The third is in a passageway you will find later that has an old cart hanging from the ceiling and 6 passageways on its sides (3 each side). In 5 of these passages, the water rushing in keeps you from exploring them, but in one, you will be able to go up into it far enough to find a chest which contains the third glowstone.

: )

The location of the glowstones CAN be somewhat random. They are not always in the same locations for everyone's games. Be sure to search all areas in a room, above rafters, under fallen columns, everywhere! YOu should be able to find the rest!

Wyv