I used the term agate clay because I thought that referred to colored clays inlaid into a slab...Maybe that's not the right term.
I colored some of my white B-Mix clay with iron oxide powder my son sent to me from France. Using different amounts in slip that I dried on plaster and wedged up I ended up with different shades. Layered slices of the colored clays with plain B-Mix, and then twisted a slice and rolled it out flat and thin. Rolled it into a slab of brown speckled Laguna clay.
But the blisters on this piece are on both that strip plus even more on the other part of the dish.
I think my idea of re-bisquing the other 2 pieces made the same way but not yet glaze fired is what I'm going to do.