Hi HuntersWife, without seeing the back a full id will be hard, but I have found one example that is similar. Sun/star bursts seem to have been very popular in the 18th century, and a lot of the examples I have looked at appear to be hand carved/decorated as opposed to moulded. Is your button about 30mm or so in diameter ?. Here is the link I found to the similar one, when the other guys get home from detecting I'm sure you will have an answer
http://www.ukdfd.co.uk/ukdfddata/showrecords.php?product=4947&cat=104
hammered