White agate is chalcedony without colour: the material is naturally translucent white, sometimes with faint grey banding. Because nothing has to be done to it to make it white, it is also the variety least likely to have been treated, which makes it a reasonable baseline for judging the rest.
Agate is a hard, tough material and this is the simplest version of it. Beads are polished to a smooth satin; the faint banding shows under strong light on some beads.