Obsidians are the standard currency on Pagan. It was agreed to use this material after long discussion what to use for a unified currency. It is somewhat difficult to determine the worth of one Obsidian, but for a rough pointer, a Kith filet costs 8 Obsidians, the scroll for secret doors 50 Obsidians. Imprinted on the coins are the seals of the four Titans (the same as seen on the cloth map), to pay homage to them.
Note that while it is the same material as the reagent used for Sorcery, Obsidians can't be substituted for the raw material, as the treatment has made the coins useless for magical usage.
During the Third Epoch, after the Age of the Necromancers passed and Tempests came to rule, the Pagans learned the value of coins - namely that a universal currency saved wear and tear on the feet. Much deliberation went on as to what should best represent this abstract item of value. Some wanted the trappings of precious metals, but decided such was too valuable for crafts and weaponry to waste on coinage. Others thought that objects from the sea, such as oysters, clams and starfish, would best honor the Tempest rulers. Conversely, proponents of toraxen hides argued that it would be sacrilege to remove such sea belongings from the domain of Hydros.
Finally, the people agreed upon shaped and stamped obsidian chips. Obsidian is no longer used for other objects, it comes from the ground, honoring earth, and can be imprinted with the icons of the other Titans to show deference to them as well. Moreover, it is possible to confine to government agencies the technology for heating raw obsidian and molding the ore into chips. Thus they can enforce a necessary, though arbitrary, value within city walls.
Those who cannot or will not craft for themselves use these obsidian chips throughout the city as legitimate exchange for goods and services.