haha! There IS actually a bonus level, and you can get to that diamond. Not all walls are as solid as they seem, you just need to find the "right" way (although I admit that it is a bit tricky).
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Ha! I finally got it! I thought I needed to Wind myself up from the bottom where the bricks look less solid. Then I tried Winding myself up both the left and right shafts and falling while pressing towards the gem. No good. Finally I discovered that the platform near the fire cannon is at the "right" height for a reason! I like your other games too. Cheers!