Black Niobium Rubber Ball Captive
The company that makes these specializes in ONLY captives and they make nothing else! The quality is amazing and the polish is mirror finish. These come with rubber balls so they are very easy to take in/out.
Replacement rubber balls are available here.
Jewelry is measured through a gauge system (see chart below). The higher the gauge number the smaller the wire is. A standard "earring" is usually 20 gauge. Jewelry gauges/sizes can vary between manufacturers. There is no universal regulation on companies to make a standard size in body jewelry. A big part of this is that some jewelry is made overseas and measured in millimeters. And in the USA jewelry is measured in inches/gauges. So many times jewelry can be anywhere from .5mm to 1mm different.
5/16" (~8mm) diameter 3/8" (~10mm) diameter 1/2" (~12mm diameter)