I believe there are earlier examples of this before Jabo. Rumor has it, back in the old days certain beer/beverage cans somehow ended up in with the molten glass (the results can range from everything between brown, dull, silver etc.) But who knows. Could be some of the marble companies of that era were getting reclaim/recycled materials from other glass companies that did leaded/stained glass. Somebody posted a pic of some vintage ones a while back.