Trading is mainly done with coins, though barter and credit letters are also available. A currency mint on the second level produces coins for all of Yliakum. Since it's a closed society, the minters only have two jobs: repair broken coins and replace the ones that leave the city or get lost somehow.
"Trias" are the best coins and the main currency. They are made of alloy, metals, or crystals that are resistant to corrosion and oxidation, which allows the Nolthrirs to take them under water, increasing their worth as a cross-race currency. The crystal mineral can easily be found in the mines and is transparent or light green in color. Magically forging the crystals produces a perfect triangle with rounded-off edges. Coins made out of steel are worth much less (1/10 of a Tria), which makes them more popular among the poor.
The "Hexa" has a value of 10 Tria; it's made of another crystal mineral, light blue in color, and has the shape of a hexagon. The red "Octa" is worth 50 Tria, or 5 Hexa. The golden "Circle" is worth 250 Tria, or 25 Hexa.
Coins of higher value are not official because these are not made in the official mint. Instead rich merchants who need an easier way to make big trades create them. The merchants use the silver coin, worth 500 Tria, and the platinum coin, worth 1,000 Tria. To make large trades like these using official currency, banks or Craft Guilds will issue credit letters to whoever needs them.
The following samples of the purchasing power of Trias will give you a better understanding of the currency:
Table 3.1. Sample Prices
As you can see, it's difficult for a farm worker to collect enough money for good equipment like swords, armor, etc. to venture into the Stone Labyrinths. Nevertheless, some try to save every Tria they can to pass on to their sons so that the son may search for a better life, which often sends them into trouble. This "killing-by-kindness" is one of the reasons that city is never overpopulated.