The diamond is in the sea. It can't be "in the grotto" because the ocean tide would pull it out to sea. This is Marie Laure's intent--why she wants Werner to assure her the little house is far enough in to be pulled out by the tide. And Werner most likely follows her wish by releasing the stone into the sea. We have no reason to believe otherwise based on the text, and the fact that he dies a few weeks later confirms that he does not have the stone. The real question here is: Why does she give him the key? She certainly wouldn't want him to have the stone (thus passing the curse onto him). Is it an implied plan to meet later some time in the future? Perhaps. A place for him to hide for the Allied troops? That kind of makes sense. In a symbolic sense, she is giving him the key to her own inner self--i.e. a world of darkness, touch, shells and water, unnoticed life, deep relationships. So he goes back and takes, not the stone which she wished to return to the sea, but the house which is the true symbol of Marie Laure. It is his memory of her--the one true and valuable thing to come out of his war experience. And the key, which is the possibility of meeting her again.
The real question here is: Why does she give him the key? She certainly wouldn't want him to have the stone (thus passing the curse onto him). Is it an implied plan to meet later some time in the future? Perhaps. A place for him to hide for the Allied troops? That kind of makes sense.
In a symbolic sense, she is giving him the key to her own inner self--i.e. a world of darkness, touch, shells and water, unnoticed life, deep relationships. So he goes back and takes, not the stone which she wished to return to the sea, but the house which is the true symbol of Marie Laure. It is his memory of her--the one true and valuable thing to come out of his war experience. And the key, which is the possibility of meeting her again.