because ages ago they used to use marbles instead of glass balls.
Counter tops, pillars,...essentially anything that is or can be ornamental.
Contradicting answers, wiki.answers.com FTL ^ Multiple similar questions containing different answers, thanks for the great answers. - NDKilla
No, they are made of glass (mostly)
Marbles originated in ancient Rome and Egypt and were made from marble (rock). However this was expensive so people tried to find alternatives and in 1846 a German glass blower invemnted a marble...