In the early 20th century, a variety of new and updated techniques to enhance photographs became popular. In this instance we see two – a decorative embossing on the bristol board and a nonstandard shaped photograph. Usually, photos were rectangular or oval, sometimes circular. I have seen other diamond shaped photos and I am certain the photographer did this to be artistic and add visual interest to the photographs.
In this case, the photographer’s information has been literally rubbed away with age. The bristol board is in poor shape – corners rounded with age and breaking, water stained, and it flakes if I touch it too much. In a frame, it would draw the viewer’s eyes to the diamond shape, focusing in on the subject, as was desired.