Zinc monuments are rare as they were made only during a couple decades starting in the 1870s.
Oakland Cemtery has quite a few of them. They are hollow inside; some say bootleggers stored booze inside of them. This monument is missing its top:
Holding my phone above it, I snapped a photo of the inside.
These monuments are also known as "white bronze" monuments even though they are mostly zinc (there is a small amount of tin in them too). I imagine the naming was a marketing move. With time, the monuments can take on a bluish hue.
Over time, the detailing holds up a lot better than in marble:
Here is a large family zinc monument with individual monuments made out of zinc for each family member.
Read more about zinc cemetery monuments
here.