The Johns Family History Association

This was an email I received explaining the types of tombstones and their strengths and weaknesses.  I found it very interesting and thought I would share it. 

       Good evening all; I am Pete Dohms, husband of Benigne who is your regular  correspondent at this e-mail address.  I am a professional geologist and this  topic has landed squarely in an area of which I have some degree of knowledge.From direct observation and over 30 years of professional experience, I can  assure you all that there is some degree of truth in what Bob has stated, but  there are limitations.  Allow me to explain.
       There are several types of "stone" that are typically used for creating tombstones.  In the early days of America (particularly in New England), slate was used.  Slate is a dark colored rock that breaks naturally along a  very strong preferential plane (cleavage).  It is also fairly soft and fairly  easy to carve.  It is also highly resistant to chemical weathering (which is  the erosional phenomena Bob was alluding to). Slate tombstones that are  hundreds of years old can usually be easily read.
       In the past 100 years, though, most American tombstones have been created  from two other types of rock.  Those are marble and "granite." In a moment  I'll explain why I put "granite" in quotation marks.  First to marble.
       Marble is a crystallized form of what began as limestone, a chemical precipitate formed as layers in the bottom of a shallow sea.  After deep  burial, and the application of heat and pressure, the limestone grains (calcium carbonate) flowed together into the larger and more crystallized  forms seen in marble.  Marble is, like slate, relatively soft and quite easy  to carve.  It is fairly easy to saw and grind, which is good since it doesn't  cleave like slate.  If you've ever been to a U.S. Military cemetery, virtually all tombstones you see in the long precise rows are marble. The  effect is quite striking. Marble, unfortunately, does not resist chemical weathering at all well. Many  tombstones from 100 years ago have become illegible, hence the "tips" that  pass around on "how to raise the inscription."  In the case of shaving cream,  in the absence of affirmative knowledge to the contrary, I imagine the surfactants (wetting agents) in the shaving cream penetrate the microscopic discontinuities along crystal grain boundaries, making those areas easier for rainwater (which in the NE USA is quite acidic) to penetrate.  Once penetrated, the acids go to work on the crystal grains, enlarging the discontinuities and exposing new marble (calcium carbonate) to chemical attack.  As the process continues, tiny flecks of marble spall off the stone, leaving a white, sugary, irregular surface behind.  In cold climates, ice also penetrates into the stone and the freeze-thaw cycle adds physical weathering to the chemical attack (recall that water expands upon freezing,  which pushes the cracks open even further).
       What I called "granite" is, in reality, a trade term for a wide variety of  igneous and crystalline metamorphic rocks that contain high concentrations of  silica (silicon dioxide, or quartz).  The other major components include  various feldspars, dark minerals (pyroxenes, hornblendes, micas), and other  rock-forming minerals.  As a class, "granite" is difficult to work into tombstones.  It is both hard and tough.  This different mineral composition  and more challenging geologic history, however, provides it with an excellent  ability to resist chemical weathering.  A year ago, Benigne & I found the  grave of her grandmother's uncle, who was buried in Northeast Pennsylvania in  1914.  Rain in this area is as acidic as any location in the country and more  acidic than most.  The fine granite tombstone that was described in his Will  looked as though it had been erected the week prior to our visit; it was untouched by the 85 years of weather it had experienced.
       Having said all that, please understand that the forgoing are general statements.  There will be specific locations where 200 year old marble tombstones are in remarkable states of preservation, and there will be cases  where 200 year old slate tombstones have dissolved to low piles of shaley  rubble, but those are exceptions to the general conditions described above.
 Pete Dohms, Geologist