Are toy marbles made of marble?

Since 1915 and the ability to mass produce them with machines, modern marbles have typically been made from glass.

Before that in the past they have been made by hand from glass, clay, polished nuts or polished stone.

The oldest known marbles were rocks harvested from rivers which had been naturally polished over time from the flowing water; use of river stones as marbles has been found in ancient times through archeology.

There have been historical finds of marbles hand made from marble, but it is not a common practice and they are rare. not my answer because I didn't make it, but this answer is the best explanation I have ever heard.