Well, flinties (and moonies) existed at least since April 1928, per Boys' Life.
Akro said they used flint glass in some marbles as early as 1926, in the booklet All About Marbles. (here's a copy)
Here's the part where I saw they mentioned flint glass -- in the Cardinal Reds: http://i119.photobucket.com/albums/o151/modularforms/Akro/1926_Akro_CardinalReds_Cornelians.jpg
They just called the Cornelians "hard as flint", at least on that page.