Some of the earliest buttons used were round: balls, spheres, etc. One variety was called a Grindle, which is named after a type of stone. These buttons could either be made over a hard bead core, or be worked over a dense wad of fabric. A conical subset of the Dorset buttons were known as Knobs and High Tops, and were among the earliest designs from that region.
While this type of button was clearly in use prior to 1700, and used again post-1800, they don't seem to have been in fashion during the 18th century.