It is not the architectural style or the method of construction that makes cobblestone buildings so distinctive, but rather the choice of building material itself.
In England and on the European Continent the use of small, smooth stones in the construction of walls, houses, and public buildings is so common as to cause little notice, but in all of North America no more than 800 cobblestone structures are known to have been built. None may ever be built again.
''Modern-day masons just don't know how to do it,'' says Delia Robinson, research director of the Cobblestone Resource Center, the repository for information on North American cobblestone masonry. This highly skilled craft has not been practiced since before the Civil War, and then only briefly. The entire period of cobblestone construction lasted no more than 35 years, between 1825 and 1860.
The number of masons who worked with cobbles during those years was small, because 90 percent of all cobblestone structures are within a 70-mile radius of Rochester, N.Y. And what these men knew they kept to themselves.
''Those masons were true craftsmen, in contrast to how we think of builders today,'' says Ms. Robinson. ''As the story goes, they simply stopped working when strangers came to watch.''
It is not clear just who the masons were or how they came to build such exquisite buildings. One theory is that completion of the Erie Canal in 1825 had left unemployed masons who knew how to make a mortar capable of withstanding weathering and water.
In the process of clearing cobblestones from the land where they had been deposited by the retreat of a great glacier 12,000 years before, the settlers stacked them as fences. Seeing this abundance of building material, the masons offered to use the cobbles to construct houses and outbuildings for the farmers.
A second theory is put forth by Robert Frasch, director of the Gannett School of Man in the Rochester, N.Y., Museum and Science Center. Mr. Frasch, the first president of the Cobblestone Society, found that American cobblestone work closely resembles similar construction going on in England in the 18th century and earlier. He thus concluded that English masons migrating to the region brought the craft with them.
But whoever the masons were, they began by building structures with 18- to 24 -inch-thick walls, using any combination of color, shape, and size of field cobbles available close by. The mortar joint was simply a wavy, slightly irregular line formed into a flat V. This makes the stones appear to project beyond the face of the wall, thus giving highlights as well as shadow.
Gradually, as each mason refined his own techniques, the structures became more elaborate, ornate, and individualistic. Smaller water-washed cobbles selected for uniformity of size, shape, and color were hauled from the shores of Lake Ontario and laid in predetermined patterns. The most complex was the herringbone pattern made of long oval stones laid diagonally, with the slope alternating in each row. Striped patterns were made by alternating rows of different-colored stones. The mortar became more decorative as well, being fashioned into raised beading along the horizontal joints and projecting V-shaped pyramids along the vertical ones.
The work was slow and exacting; it took as long as four years to complete a single farmhouse.
In the earliest buildings the walls are made entirely of cobblestones and mortar. Later ones have a cobblestone facing on a wooden frame, the mason's time being devoted to creating a decorative facade rather than to constructing the building itself.
The secret to cobblestone masonry is the mortar. Each mason made his own mortar using sand, water, and local limestone, which he processed into lime. Six months or more was required just to prepare the lime.
Discovering how to make a comparable mortar today is but one of the goals of the Cobblestone Resource Center. In addition to collecting pictures and histories of each of the cobblestone buildings in North America, the center's ongoing research program provides information to homeowners on the latest in restoration techniques.
''It's very difficult to find a mason even to repair cobblestone structures, '' says Ms. Robinson. ''Most just turn you down flat. We'd like to assist young masons in learning the craft. Not only would they have a specialty that would guarantee them work, but it would guarantee the preservation of these rare buildings as well.''
The resource center and its sponsoring organization, the Cobblestone Society, are housed in the Cobblestone Museum, one of six restored cobblestone buildings in the village of Childs, N.Y. The buildings in the village, a short distance from Rochester, are open to the public during designated visiting hours.