Are pentagons and hexagons a different geometric form from circles?Yes.
A circles has no/infinite straight edges. A pentagon has five.
But Greeks and Italians are not a different race from Scandinavians. To modern Americans, anyway. Plenty of people once thought otherwise. Do you see the difficulty?
Perhaps if I had used colors as races and red, yellow, and blue for the peoples would that have a made a difference? Does the fact that there are intermediary colors invalidate the existence of prime colors? Nothing in nature exists an a prime color. Everything is a variation. But the difficulty of determining if teal is more like blue or more like green does not mean that there is no true green or true blue. Determining if an individual fits one racial category or another is difficult. Like colors or certain animal species, these determinations are based on judgement and definitions. But given that, one cannot deny that there are objective and quantifiable differences between blue and yellow even though there may be intermediary forms. You cannot also deny there are objective and quantifiable differences between Scandinavians and Sub-Saharans even though some may try.
But keep in mind that those differences are of no concern to government. We should all be equal under the law. The government has no right to ask me my "race."