Maby a new fad if not already thought of

Not sure if this is how it works already but when you take it to the pavment and start sparkin is it the frame or is it pieces of metal atached to the botum. if it is metal pieces on the botum why not put on some different types of metals, because alot of them spark in some prity cool colors. No critsisem if thats a stupid question/sudjestion