4mm Swarovski Flatbacks also known as ss16 Rhinestones are the most popular size for these highly polished cut glass multi faceted crystals. They have a high lead content which makes them heavier than standard glass crystals and also increases the index of refraction, which in turn makes them sparkly like diamonds when they come into contact with light. They are used extensively in the fashion and jewellery industries throughout the world because of their diamond like qualities. They add sparkle and glamour to evening and day wear as well as jewellery and countless accessories.