Original post by: Henry H ,
These are pieces of scale, most likely calcium; they are formed when minerals in the water react with the metal of the boiler. Usually they adhere to the inside of the boiler but they can break free and plug the steam wand, among other things. Descaling the machine, and continuing to do so on a regular basis, should remedy this issue and prevent it from happening again. This is normal maintenance for any espresso machine.