8d (eight penny) nails are made from 10 1/4 gauge (oh how I detest those English/American standards!) steel wire. 10 1/4 gauge wire has a diameter of 0.131 inches or 3.33 millimeters (metric - I can relate to that!).
Many different size nails have 'ribs' on them.
They are to make it harder for the nail to be pulled out.
A 'spiral' ( ardox ) nail is even harder to pull out.