What kind of rock is this, click link to see picture?

Question

http://www.flickr.com/photos/22936548@N08/2581891308/

Answer 1

It looks to me to be a piece of limestone or chalk; however, since I cannot lift the piece and examine it, I could be wrong. Is it easy to scratch or crumble? Where did it come from? Does it bubble if exposed to vinegar? How dense or heavy is it? Are there grains or crystals in matrix? Please respond, if you are able to find this knowledge (e.g. if the rock is in your possession).