Seed sugar crystals are rolled in a sugar mixture inside a large rotating drum for several days or weeks, depending on how thick the layer needs to be. This process deposits a thin, uniform layer around the seed crystal, which gradually builds up, much like a pearl, to produce a solid spherical sugar core. Silver is added to the mixture and coats the ball. The coating is aluminium and there is no proven risk in eating small quantities you find in cake decorations.
Source : The Hindu