"Quoins" are the exposed stone pieces that you sometimes see stacking up only on the corner of a building. They sometimes look like a zipper applied to the corner edges of a structure. And their use today is odd and usually fake and is trying to allude to traditional masonry construction and presumably the sense of durability, solidity and timelessness that implies.
Quoins provide a kind of emphasis, a visual boldness, to the corners of a building and tend to make the building feel more solid, more object-like. However, like so many elements of architecture that appear to be merely stylistic touches, they have an origin in a construction technology.
Quarrying stone has always been a difficult and expensive proposition. Making a stone building out of the stone that are scattered around the field and forest is a much easier proposition but results in a random rubble type wall. When that random rubble wall has to turn a corner, the stone of differing sizes and shapes create a visual and technological problem. Because of its ragged line it collects water, because of the use of small, varied stone, they easily pop off the corner when exposed to the elements from two sides. Quoins of cut stone were used to contain the edges of stone walls and help solve these exposed corner problems. Being cut stone they stack nicely and cleanly on each other and their consistent size and shape they solidly and securely hold the corner true and vertical.
However, this technological use of quoins has long been forgotten and they are merely stylistic touches now applied without much subtlety to buildings. You can see fake stone quoins, face brick quoins, wood quoins trying to look like stone quoins, and best yet, EIFS (fake stucco) quoins, in buildings all over the country.
"cast" stone quoins on parade
wood "stone" quoins; actually as you can see, quite an old fakery often used by New England ship captains to give their wood houses a sense of class
fake stucco and foam quoins
Let's all try to be a bit more judicious in our use of quions. Let's not fake them using stucco and foam, and certainly, let's only use them on masonry buildings. They are not simply another style product to be plucked out of Ye Olde House Stuff catalogs and glued to a building.