It's an interesting question.
First, under the act, miners, cutters, and polishers are not covered, so the act doesn't apply to them. That said, government certainly wants royalties from the resource that comes out of the ground, so I imagine it would be Natural Resources Canada or another government agency that would be monitoring that sort of thing.
The vast majority, in fact to my knowledge all the rough that comes out of any mine anywhere in Canada leaves the country and goes to a sorting facility in Antwerp, London, or somewhere else, and then some of it is returned. I could be wrong on this, but I think even the small amount of rough that is cut and polished in Canada, which is diminishing all the time, has left the country and comes back. In there somewhere would be another government agency, I think, that would have oversight on it.