Nice marbles!
The little flecks that you call “Lutz” are actually un-dissolved bits of minerals that were used in the recipe mix. We, as collectors will call this “Aventurine” commonly. The green colors of all makers have this more commonly than any other color.
Lutz is associated with old “German” Handmade marbles for the most part. Contemporary handmade marbles may include some “Lutz” as well.
Marble—On!!