From the Wikipedia page 
The mystery stone from Meredith , Lake Winnipesaukee is an alleged out-of-place artifact (OOPArt), reportedly found in 1872 while workers were digging a hole for a fence post. It is a carved stone about 4 inches (100 mm) long and 2.5 inches (64 mm) thick, dark and egg-shaped, bearing a variety of symbols. The stone's age, purpose, and origin are unknown. Seneca Ladd, a Meredith businessman who hired the workers, was given credit for the discovery. Upon Ladd's death in 1892, the stone passed to one of his daughters, who donated it to the New Hampshire Historical Society in 1927. The stone is currently on exhibit at the Museum of New Hampshire History.
Carvings on one side of the stone show an ear of corn and several other figures. The other side is more abstract, featuring inverted arrows, a moon shape, some dots and a spiral. There is a hole through the stone, bored from both ends with different size bits (1/8 inch at top and 3/8 inch at bottom).
The American Naturalist of November 1872 suggested the stone "commemorates a treaty between two tribes." A letter to the New Hampshire Historical Society in 1931 suggested it was a thunderstone. The writer said thunderstones "always present the appearance of having been machined or hand-worked: frequently they come from deep in the earth, embedded in lumps of clay, or even surrounded by solid rock or coral."
A borescope analysis of the stone's holes was performed in 1994. In a 2006 article by the Associated Press, state archaeologist Richard Boisvert suggested the holes were drilled by power tools from the 19th or 20th century. Boisvert reported, "I've seen a number of holes bored in stone with technology that you would associate with prehistoric North America. There's a certain amount of unevenness ... and this hole was extremely regular throughout. What we did not see was variations that would be consistent with something that was several hundred years old." Scratches in the lower bore suggest it was placed on a metal shaft and removed several times.
Analysis has concluded the stone is a type of quartzite, derived from sandstone, or mylonite.