That's a pretty marble, Chris. I think that the word "agate" was initially adopted by American marble companies since it was associated with the quality stone marbles from Germany, which were something they wanted to replicate with glass.
As for minerals in marbles, gold lutz and mica were used in early German handmades many years ago. I think aventurine is sometimes purposefully produced and sometimes just results from reactions of certain colorants used in he manufacturing process.
I am sure others could add more to this discussion.