[Strategia_In_Ultima]

Most expensive? Naah. Diamonds are IIRC actually one of the most ubiquitous precious stones there are, it's just the fact they used to be so rare (and of course their sparkly shiny look when cut, and the fact that it's the most dense naturally occuring material known to date) which is why they're expensive.

Then again, my source (old space strategy game called Fragile Allegiance) may not be entirely accurate in this time period, so I might be completely off the mark here.....