Stud caps made of 925 sterling silver, plated with rhodium, which gives the silver the color of platinum. Rhodium is a rare precious metal of the platinum group. Applying a rhodium plating to the jewelry reduces wear and increases the hardness of the jewelry, protecting it from scratches. Platinum group metals do not enter into any chemical reactions with most of the aggressive chemicals known to mankind, and therefore are completely hypoallergenic, that is, they do not cause allergies.