Your medal is a "miraculous medal" or "medal of immaculate conception". They are hard to date because they have been made essentially similarly since 1830, when Mary was purportedly seen in this pose. Many bear this date, but it does not mean it was made then (they are still made this way). Originally they were all oval in shape, as I believe Mary allegedly dictated to the gal who saw her. So, somewhere along the line they got a bit loose with this detail. It could be assumed to be about as old as your marble, perhaps. It's a glazed clay marble, right?