This is such a fun question to answer! While sequins today are usually just made out of vinyl plastic and dipped in a metallic coating, they haven't always been so. The earliest sequins actually go back to ancient times, to none other than King Tut himself.  Apparently real gold sequins were sewn to this pharaoh's clothes, ensuring he had enough money in his afterlife. Other cultures used brass or other metals to create a sort of armor for clothes, such as cultures found in China and even 17th Century England.  As they grew more fashionable in the 1930s, gelatin became the popular choice (though they would dissolve in the rain). And today, while some brands do splurge on luxe metals, plastic is now the go-to.