There are a lot of marbles made by a variety of companies with bubbles in the matrix. Peltier is known for them.
When the Japenese/ asian marble makers invaded the USA markets they were using recycled glass in the clear matrix.
This explains the lack of crystal clear glass which is a batch recipe(made from a glass recipe), or very fine cullet glass that was purchased by marble makers in the USA from a variety of other glass makers.
MarbleāOn!!