How to determine how long ago a rock was broken?
There is a technique called "Exposure Dating". Using e.g. cosmogenic nuclides (they are produced in the first metre or so of the surface of a rock which is exposed to cosmic rays, i.e. on the surface of the earth), it is possible to estimate the amount of time that the rock was exposed. 

With some assumptions, you can get an exposure age of the fracture surface (or at least some information about it) since the fracture surface was exposed to cosmic rays only recently, whereas the rest of the surface was exposed to it long before.

The specific sample is probably too small though. This technique is useful to date the retreat of glaciers (they shield cosmic rays from the underlying bedrock), fault scarps, or the time a meteorite spent flying in space from its parent body to the earth.