Feel free to add this to any FAQ that gets created, just give me credit *smile* but feel free to change anything up/formatting/etc. :)
Gems and Metals are various "extras" that are scattered around the Sim's world, it can also be a way to get a little spare change, although doesn't seem to be terribly lucrative as the more rare ones are the only ones that give you any decent payback, and well they're rare for a reason... you don't normally find them!
So I tried to make a table of gems/metals and what not, then I realize they all have a weight associated with them, and that also affect their dollar amount, so rather than say "You shouldn't cut gem X" I simply found out how much a particular cut/smelt added to the value of said rock, and from then I can mathematically compute what the minimum value of said objects should be to give it a particular cut.
Why does this matter? Let me do an example I found a particular piece of silver ore that was worth 30. If I smelted it, -40, the ingot I got back was worth 52. Simply right? The net gain was 12, unfortunately this actually ends up being an overall loss. I could have gotten 30 if I sold the ore out right, so I actually ended up losing a value of 18. In fact it turns out you should NEVER smelt silver, as it will always be a net loss of value. Now unless there's a way to smelt a whole bunch at a time (which I have not found out) you'll need to have an unusually large piece of silver ore in order to gain value via smelting, because smelting has a fixed cost, 40 simoleans.
So finding a TON of ore and uncut gems I figured out the value increase.
So the way this "chart" lies out the action done, then the cost to do said action, the value of the processed, the minimum value to be net increase in value.
Smelting, $-40, 175%, $54 So for smelting, it costs $40, it increases the value to 175% of it's original value, and the ore needs to be worth $54 or more in order to increase the amount of money you'd get to do this over selling the ore outright.
So as you can see the better the cut the more expensive the needed gem material need to be, in fact there are quite a few gems you'll find that do not meet the minimum ($40) for an emerald cut, Topaz comes to mind, so just sell the whole lot. In order to get the better cuts you need to do a lot of the lower ones, so to get oval you need to cut a bunch of emerald gems, to get Pear you need a bunch of oval cuts etc, so in this case you might use those Topaz gems even though they won't get you any net value increase (in fact they will most likely be a net loss on the cutting process!), just to get that next level if you desire. Not sure if there's anything beyond brilliant, wandering around town is really boring, even with the collector locator special.
Lastly, occasionally you'll get a message that the gem/ore was really a gnome... rather than grumble and add it to your lawn collection, drag it over to the sell box for a VERY nice surprise ;).