What is called "aventurine" (aventurine is actually a mineral) is formed one of two ways. The far most common way is by accident by adding too much colorant to the batch (its added by the shovelful) in the form of various metal oxides which crystalize as the glass cools. The far less common way in intentionally - as Peltier did with a number of their better marbles. This latter approach was a bit more costly and created a very dense crystalline patch.
Over-saturating the batch a bit with colorant wasn't uncommon, especially it seems with green glass.
As I've said before - there wasn't a big wooden barrel at the marble factory with the words "AVENTURINE - USE SPARINGLY !" printed on it with some magic powder inside. Marble making wasn't rocket science and vintage marbles needed to be made for tiny fractions of one cent.