[Kristoffer O]

Gold has the alchemichal prperties of the Sun, and the sun is made out of fire. Thus you can give any base metal the properties of the sun if enough fire magic are used in the process.

Earth magic can be used to transmute one form of metal into another, in other words lead into gold.

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Also, as a side note, gems are just a conventional way of saying 'condensed magic' (vis, in ars magica terms). They are not supposed to be sold. If you don't want to loose the RPG feeling you can say that magical gems are small enough to not be worth anything to a non-mage.

Edit: Early on in Dom-ppp development, only fire gems were usable.