Here's a use for marbles I once read about in a newspaper. I've mentioned it before but now I have a source:
The Jeweler's Circular, Volume 77, Issue 2, p. 47, January 29, 1919
https://books.google.com/books?id=kzUcAQAAMAAJ&pg=RA7-PA47
Wounded soldiers were trained to cut diamonds. Initially they practiced on marbles.