Freehanding a smooth plane with a mallet and a chisel, or just a hammer stone, is difficult on large surfaces.
A flattened copper strip with sand as an abrasive would saw stone surfaces, bronze strips with jagged teeth would saw wood. These surfaces would not necessarily be triangular, merely flat planes.
These perfect shapes were given their own description due to being obviously distinct from a chipped surface.
Common usage beginning in the 1500's gave the term its present day meaning of a sawn and polished glass triangle.
A shiny rainbow overwhelmed any previous meaning.