I never would have thought about this. However, after reading about it, the idea makes perfect sense. Scientists believe gravestones may hold secrets of how the Earth's atmosphere has changed over the centuries, and those scientists are now asking for the public's help to read these stones.
Little by little, atmospheric gases dissolved in raindrops cause the marble in gravestones to erode. As such, headstones can serve as diaries of changes in atmospheric chemistry over the years due to pollution and other factors.
By gathering data from marble gravestones of different ages across the globe, scientists hope to produce a world map of the weathering rates of these stones. They are asking volunteers to take measurements using simple calipers and GPS, following a set of scientific protocols that are explained online at the Gravestone Project. They can also log data into the scientific database at the site.
The Gravestone project may be found at http://www.goearthtrek.com/Gravestones/Gravestones.html.
An excellent introductory article by Charles Q. Choi may be found at http://www.livescience.com/environment/091209-gravestone-climate.html.