Bloom or Blooming
Most nitrile and neoprene rubber materials when stored undergo a process called blooming. Bloom is a milky dusting of dry powder on the surface of the rubber. Usually this is caused by unused vulcanizing agents migrating to the surface of the rubber. This blooming is entirely superficial and will not affect the performance of the rubber in any way. Should you find the gray color to be unacceptable, you can wash the o-rings in water or light mineral oil to remove it. Since blooming is entirely normal and does not affect the function of a rubber seal, it is not considered a cause for defect or rejection. Likewise, it is not considered a contaminant in the rubber material.
Simply put, a cross section is the thickness of one side (or the wall) of the o-ring.
The inner diameter of an o-ring is the space inside of an o-ring from one side across to the other. To measure the inside diameter (ID) - You can do this by either using a ruler or calipers to measure from one inner edge to another.
The outer diameter of an o-ring is the total distance from the outside of one side all the way accross to the outside of the other. To measure the outer diameter (OD) - use a ruler or caliper to measure from one outer edge of the O-ring to the other outer edge.