I would doubt it is limestone for a couple of reasons.
1. Limestone is a sedimentary rock, compressed silt really.
2. Being only compressed silt it is not all that hard and would crumble when trying to work it to an edge, or at best would flake off for a very uneven edge.
I would lean towards, bone, antler, flint or chert. To me the design strikes me more as a knife then a hide scrapper. Generally scrappers were of a broader, half moon shaped, edge and fit the entire palm of the hand. But then again I am no expert. Quick way to check if it is limestone is put a small drop of hydrochloric acid on it. If it fizzes then it is limestone. Rinse in water and the effects will not be seen.