So, I just had to see what was inside some marbles that I've had for over 10 years. I chose one that was one of the smallest (and slightly out-of-round), put it in a vise and hit it with a hammer a few times. They're quite hard, too. Inside looks like and feels like glass. As I stated in the title, these are HEAVY for their size.
LEFT PHOTOGRAPH: The middle marble is the one that I hit with a hammer a few times. I know that the marble in the middle doesn't show much of the internal glass, but it is there. The marble on the far right is white-green with a strip of metal on the surface. The far left marble is the pink-blue-tan-white one.
RIGHT PHOTOGRAPH: The two marbles on the left (now looking at the right photo) are the same two in the left photograph. The far right marble (blue-brown-white with red-green-white spots) is one of the marbles in question.
For comparison in weight, I chose one of the marbles in question and then found a Marble King of the exact same size. They both measure 25/32". The Marble King weighs 32 grams and the marble in question weighs 61 grams, almost 2X the weight of the same size Marble King.
Let me know what you think. Thanks!