[ZenGum]

Marble is limestone after metamorphosis, i.e. subjected to heat and pressure. Same chemicals though.

This looks to be somewhere between the two - slightly metamorphised limestone, or poorly metamorphised marble.

I was going to facetiously suggest that you splash a few drops of hydrochloric acid on it and look for bubbles, this being a standard test for limestone, but I think Lamplighter has a more practical idea.