Don't go near the stairs or you can not get the treasure. Soul Gem - 10,000 - Ch 4-1 - When you have to run away from the large monster called the Popokarimu, (the bat-like monster) instead of running, stand your ground and fight.
once that is done, you can find the gem between the stairs on the first floor in the main room during the fight with Jill. Heart of Africa - 10,000 - Ch 5-3 - During the fight with Wesker, you must inflict sufficient damage to make him end the fight early.
Here is a short list of the most valuable treasures that you might not find during the regular course of the game.
