In the last episode of the 5 that brought Bubba to the 20th century (and they could of left him in pre-historic times and it would of been just ducky with ME) "Ali Bubba's Cave" the steam cannon shot diamonds to the island Mr. McD owns. Why JUST diamonds and not rocks? Because it did shoot rocks and diamonds- but the rocks got powdered to find dust by the blast and only the diamonds were tough enogh to stand being blown up like that.
And you know what else? Flinty saved Mr. McD all sorts of money (unintentionally). Thanks to that steam cannon the boys (10 year olds?) were forced to invent, Mr. McD didn't have to dig up the diamonds.