In Supergirl season 5 episode 6, 'Confidence Women', Lena wanted to keep Lex from finding the element himself as it would allow him to vaporize Kryptonite, thereby killing Superman and harming all of humanity.
Andrea found the medallion first and hid it from Lena. Also, Lex never found it, which was Lena's objective. So what's the problem?
How would it have been different for Lex if Lena had found it?
Just to emphasize that this was a pivot point in the relationship between Lena and Andrea. They were good friends, and Lena shunned her for years because of that. She blamed what happened to Lex almost solely on that.
It continued to the next chapter, where Andrea explains that she really felt bad about it, but she wanted to save her father. Lena didn't accept it. Furthermore, she threatened to kill Russel if she doesn't get the medallion.
All of that for what? Just to keep it is a bauble, a reminder of her mother.