I found this:
The halo thing is actually pretty intricate. There are not only plain round halos, used to signify saints, there's also the cross within a halo, used for Christ; the triangular halo, used for representations of the Trinity; and the square halo, used to depict unusually saintly living personages.
Here is a very rare hexagonal halo that surrounds the head of Hope one of the three virtues. This halo distinguishes the wearer from ordinary heavenly beings, who have circular halos, and from earthly beings who wear square halos if they wear any at all. This halo seems to set her apart as a person representing an idea or value.
The apostle Paul is seen here surrounded by a square halo that shows him still living on earth at the time the artist wished to portray him. This type of halo set the wearer apart from his or her earthly comrades because the wearer was a more spiritual being, yet they were undoubtedly mortal.