Graham’s answer to “So, what really happened to the diamond? Was it answered and I missed it, or was it left up to the…” > Likes and Comments

I think it was because then she has no further evidence of having possessed the treasure which the German officer was hunting. She knows Werner will not betray her, but she does not know what she will encounter when she is on her way to safety. Somewhat self protective, and also to end the torment which the jewel has brought to those who've sought it or kept it. I think she also sees it as something that divided her from her father.