The person who was wondering what this object was that was lying with the six bullets that John gave Jim, and what significance this object has, I found the answer to your questions, and this is ONE OF THE CRAZIEST THINGS I LEARNED FROM THE MOVIE

After searching the screenshot on the Internet several times, I determined that maybe it was an oyster shell (it doesn't look like anything else) and then went to look for the meaning of this object

The meaning of an oyster shell as an attribute of Venus and Aphrodite doesn't fit here, so after digging even deeper, I found this:

Translate: «Also in the 17th and 18th centuries, when oysters became famous as a natural aphrodisiac thanks to Casanova's memoirs, the oyster shell could be perceived as a sign of fleeting and questionable carnal pleasures»

I don't think I need to explain what an aphrodisiac is and what it's used for