A small square or diamond-shaped piece of glass in a stained-glass window is called a quarry. Quarries are often repeated in geometric patterns, making them useful for backgrounds, borders, and large areas of glazing.
Many medieval windows used quarries decorated with painted motifs. These could include leaves, flowers, stars, heraldic symbols, or simple geometric marks. Repetition allowed workshops to cover broad sections efficiently while still giving the window a richly patterned appearance.
The word can cause confusion because quarry also means a place where stone is extracted. In stained-glass vocabulary, however, it refers to the shape and function of a glass pane, not to stoneworking.
A quarry differs from a medallion, which is usually a circular or framed pictorial panel. It also differs from a lancet, which describes a tall, narrow window opening rather than one small piece of glass.