Blackbeard's cave

Have got to the end, solved all clues and found everything except the blue gem in the cave so I can't move on to the treasure. Help please

The only blue gem I remember was in The Lost Tomb and it appeared in small door in the wall on the right side.

My list has a BLUE GEM but cannot locate so cannot move on. Is this a game glitch?

blue gem is above middle chest to the right...must use brush to revea.