Fieldstone was a common grave material choice among struggling settlers. The stones were found while tilling or clearing the land. Many were laid unmarked, some sported symbols or the name and age of the deceased.
An excellent example of fieldstone markers exists in Grande Pre, Nova Scotia at the Acadian Burying Grounds. A Herbin Cross commemorates this National Historic Site and the burials there, which are dated between 1680 and 1750. 
Sandstone & Slate
During the 1600s, ample supplies of sandstone replaced fieldstones in Colonial North America, because of its durability yet it was still soft enough to carve easily. 
Some sandstone markers remained so well preserved that each chisel mark can be discerned in the carving, but the vast majority of these stones delaminated and crumbled into dust. Delamination occurs when moisture gets between the layers that make up the sandstone. As it freezes, expands and re-freezes the layers flake off. This process is also referred to as “spalling.” 
If you find a memorial tablet of slate, chances are the stone dates between 1796 and 1830. The smooth texture of the material made it easy to carve and it was often highlighted with white paint or gilding. Most shapes resembled doors representing passageways from this world to the next. 
The only problem with slate: being slightly porous it has a tendency to flake, split and crumble over time destroying the carved inscriptions, as shown in the photograph (below) of the Old Pine Cemetery in Philadelphia, Pennsylvania.
Marble was later discovered to be more durable than sandstone and slate and slowly became the popular stone of choice.
 The Herbin Cross, the Grand Pre National Historic Site and the Acadian Burying Ground [Internet] accessed 16DEC2009
 Volrath, Kathy. A Monumental Undertaking: Mathias Monuments, ChamberWORKS (October 2008, Westminster, MD: Carroll County Chamber of Commerce) [Internet] ChamberWORKS October 2008
 Veit, Richard F et al. New Jersey Cemeteries and Tombstones: History in the Landscape. 2008, Rutgers University Press
 Milk Row Cemetery Guide, Somerville, Massachusetts (2002, Somerville Historic Preservation Commission)