A convincing fake - but it won't unlock the key for us. A swap is in order.
Geron had an idea - you can re-read it by examining your fake key or checking the signpost.
Something you may not have noticed is a hole in Harm's cabin - right above the key is a small hole in the window.
If we could get Nuri level with that hole, she'd be in the perfect position to make the swap.
Nuri can't fly, so you'll need to find something on the outside of the ship that can hoist her up to the window.
Something that can be hoisted up and down at will.
You'll find it on the ship; we've only been ducking in and out of Harm's cabin thus far, so try getting a good look around.
Certainly - it's tucked out of the way, but you can actually reach the ship's by clicking above Harm's cabin.
You'll find what you're looking for there.
A lifeboat - it can be moved up and down at will by using the cranks near the ship's wheel.
Ask Nuri to get inside the lifeboat, then give her the fishbone necklace.
You can then bring the boat up to be level with the window, and talk to Harm to distract him.
Well done! All that's left now is to actually steal the prism.