This can be one or two things. Reticulation or orange peel.
Reticulation occurs when the surface of the metal begins to melt. It looks like wrinkles and happens during the heating process when you anneal and you get it too hot. To avoid this, anneal in a dark room and remove the flame to see the color of the metal. As soon as you see a dull red color, let it cool to a dark heat and quench in cool water. This is a general annealing rule for any non ferrous metal . Silver is the best thermal conductor so you will notice it heats up much quicker than any other metal and easier to reticulate.
Orange peel occurs when the surface of the metal becomes weak and starts to buckle. I have noticed this is more prevalent in the pure metals such as silver, gold and platinum. When the ring is reduced more and more or when it is stretched more and more. The more it is manipulated, the more orange peel will occur. Alloyed metals seem to not have this issue. I have not been able to find a resolution to this as of yet. Just let your customers know this occurs more with the pure metals and is unavoidable.