I think the easiest option is to get a dog and have it's digging skill high. Have it keep fetching metals and you'll eventually get it. Or, you can do inventing and have your sim invent the underground scanner thing, it'll tell you if there are metals, gems, seeds, etc below an area and dig. That's probably the slowest way to get it.
Sorry I can't be of more help but it's the only way I know of to get what you're looking for.
Good luck.