Size Does Make A Difference. In marbles, like other things, size makes a difference in the value. The larger the marble, the higher the price. You said that it was about the size of a penny. This would be about 5/8" or so which makes the book on it worth maybe $300. These marbles range in size up to 1-3/4" and those larger marbles command a price of as much as a couple thousand dollars, book value that is. Here is a comparable from ebay but it is nowhere's near as nice as yours. They say that it is 8/10 of an inch in diameter. I would be as accurate as you can in describing the size of your marble as this will allow you to sell it for a higher price if it is indeed larger than 5/8". Good luck on a great find.
I agree,but the price I was looking at was at auction.That's also why I said you should contact a expert.Marblebert who sales on ebay might be a person to contact.I would also look into robert blocks marble site for info.