There's no indication that anything but Sandman and the locket remained after his transformation.
You can see from this snapshot that his clothes are being transformed to sand, but it's too big of an assumption to say that anything other than his clothes (and body, obviously) were transformed. I'd say the only organic matter that remained was Sandman himself by his sheer force of will. We only see his organic molecules transform into living silica. The scientists only mention molecules, but this wouldn't necessarily count metals which are distinct by being a collection of Ions and Electrons.
A molecule is an electrically neutral group of two or more atoms held together by covalent chemical bonds.
The remaining metal may have been small enough not to be seen once he has transformed, hidden by the extra 'sand' added or simply tossed to the side in the ensuing mini-sandstorm.
Regardless, he doesn't have an emotional attachment to his zipper or buttons. It's clear metal wasn't transformed.
In the original comics, it appears he never full transformed, and only slowly learned his 'affinity' for sand.