Okay cool, thanks for the response. I mean this question mostly in the context of non-artifact crafting and decorating, but it sounds like it applies the same way (though obviously you cant encrust with rough gems normally). Cut gems just get a 20 base value instead of the usual 10 when encrusted onto an object. That's good to know! (and something the wiki doesn't really explain)
Gems are double value material when used as decorations, that's why I emphasise that whether they are cut or not doesn't matter for decorations - again, base value of item doesn't matter for decorations, only the material itself. The same is with platinum for example - platinum nuggets, blocks or bars all give the same value when used in decorations, though apart from strange moods you can use only bars. When you use an item for decoration, its previous form is lost and doesn't count.
Gems have special modifier so you don't lose value. Though there is still one mechanism where you can feel "cheated" when dealing with gems. Large gems have base value of 10, so if made in the lower qualities than masterful, you lose value you could get if the gem was cut normally, instead of into large gem.
Another modifier is for encrusting ammo (or was, I'm not sure if it wasn't removed, I don't encrust ammo), lowering value.
I also get the impression that gems are the only decoration that keep their base value in this way? A bar of metal just gets a base value of 5 multiplied by its material, but metal studded onto an object gets a base value of 10 and a quality modifier, right? Which would mean something like a metal cap (base value of 5) would be a less valuable use of metals than studding other objects (ignoring the utility of the cap itself).
Mostly this is just highlighting (to me) the significance of actually using your materials, if your goal is to generate wealth efficiently. It seems much more useful to encrust the gems you've already found than to just mine for more gems and leave them in a bin.
Bars have base value of 5, decorations have base value of 10, plus they can have quality modifiers, so decorations are at least twice as expensive as bars they are made from.
Caps have actually base value of 10 (as do most armors and weapons in v50, except these made in pairs, which have base value of 5). So in their case it is more valuable to send them to whoever has higher skill - armorer or the metalcrafter. However, there are cases of greaves, mail shirts and breastplates. They still have base value of 10, but require more bars than one (2, 2 and 3 respectively). On them, you can lose value easily. Though I still would make them instead of decorating with metal - to train armorsmiths.
But the best if you want to make value routinely from metal is to produce bolts. Stack of 25 steel bolts cost from 750 of base quality to 2250 of masterful quality, compared to 150 of the steel bar. For comparison, any item or decoration made of steel, masterful quality, costs 630. This also trains weaponsmiths, so double win.