A pile of limestone at Conklin Limestone Company.

A close-up of a piece that weighed several hundred pounds with about a 20# piece of pure limestone resting on top.

The large piece had some other colors in it.  My guess is that this is what is called marble.

I read that a drop of vinegar on limestone will fizz.  I brought home about a 5# chunk to observe this reaction.  Might have to carry a small bottle of vinegar with an eyedropper to test stones :o)