Ball Diameter - is the size of the ball or bead on your jewelry.
Diameter - a straight line segment that passes through the center of a circle.
Length - the distance from one end to another.
Gauge - with regards to jewelry, its the thickness of the jewelry.
When measuring a diameter of a circular barbells or of a ring, you should measure it inside the ring or of the circular barbell but if your measuring a curved or a straight barbell, you should measure it from the first end to the other end.