Edited by Kym Huynh
Just after you collect the last chest, go left and drop down to area below, push boulder from the right down the gap to left past ladder, drop after it, secret passage just to left of dropped boulder under broken platform.
Secret area above exit door on right jump right from platform and crawl/walk right to gem.
When you are told to run fast past falling boulders, ignore! Wait and climb up/left over boulders – in left wall – secret passage to gem.
First gap, you drop down, jump up and left secret passage to left to gem.