Hi, when you got that stone key you have to go back down the tunnen, avoid the light, swim across the pool and get out on the opposite side. Enter the room and turn right. You (Indy) must go to a pedestal at the end of the room. Be carefull, the dark blue diamonds on the floor are triggers that shoot darts from the walls. Carefully make your way acros the room. Go left of the rubble in the center of the room to the stairs. The stairs are equally trapped. Climb the pedestal, jump to the platform at the end of the room and use the stone key in the keyhole on the left.
Jump into the new hole in the floor.....
This is the text out of the Canofworms walkthrough, I hope it will help you to go on now.