It seems that the marble is in focus but the shell is out of the field of focus. Inching a tripod back and forth and taking a shot avery few cm. would have given you at least one where everything is in focus--front to back and top to bottom.
I have to admit I have a prejudice toward highlight and lowlight impeccably focused (unless it's obviously a depth of field photo). I believe the shell to be ever so slightly out of focus but otherwise technically perfect.
This is one of the photos that really makes this challenge difficult, so good it is a a real pearl! It might be interesting to see what type of reflections one could get, but I cannot think of what could be done to better this one.
To me this is very sexual, a bit like the shiva linga from Hinduism that combines symbols of male and female genitals. However, it also unites something organic with something that appears artificial, although both things have their own geometric beauty. It's a very strong, symbolic photo. Well done :)
This is a nice shot! It may be my eyes, but I think there is a slight tilt to the right... I think this shot needs perfect alignment to work at it's best potential :)