I need some idea's for my son-in-laws 50th Birthday he is an Opal miner so I would like some idea to include Opal mining

How many does it have to feed?
You could do a mine shaft/cave and a figurine of a miner. Then have opals outside the cave and/or the miner holding and opal...
I made a fairly simple BC cave cake (unfortunately I froze it and the color and texture looked better before) with a caver figurine:
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1521671&sub=1521707