The Ss'tiss have three different sorts of coins. Aside of using these they do however also stick to using gems as a sort of payment. 

In the cities coins are much preferred because not all merchants are able to determine about the worth of a gem. In large warehouses and the like it is possible to pay with gems as well but in the usual inns, taverns, stores etc. one usually has to pay with coins.

Among the nomads coins are rarely used - among each tribe there are a couple of Ss'tiss having experience with gems, gem-cutting etc. They are perfectly able to roughly determine about the quality of gems. 

There are official exchange booths in each of the cities and villages where especially the nomads can change gems to coins vice versa. (and where also different currencies can be changed of course...)

Since the metal for creation of the coins is mostly gained in the mines somewhere in the Chrithsh'zziths Athresh (Falcon Nest Mounts) - about a day's travel southwest of the dry riverbed - it makes sense to mint the coins right there. 

Copper is also mined in the small mountain range with the volcano. Heavily guarded caravans ship it to the mint is need be. The area around the mines is protected by female Ss'tiss warriors who are able to handle their sunblades as easily as they rip apart a rat with bare hands. 

A stone tower guards the mint and the nearby camp of the warriors. Everly year the guards are replaced - they are chosen from all the different tribes and those warriors that have proven their power and loyality to the government. The eldest of the largest tribes may recommend certain warriors but the government is not restricted on those.
 

The coins used in Tahnn are the following:

Gold: "Falcon Eye" = Chrithzesh (its color resembles the eye-color of a certain kind of falcon that lives in the mountains)
Silver: "Silver" = Thrincz 
Copper: "Fire" = Lyssth  (its color very much resembles that of the dancing flames of fire)

All coins have an intricate, hard to copy pattern along the edge. Between relief-like intertwined lines the word "Ssith'sa" - my God (Sun) - is "printed" into the metal. 
The golden and the silver coin have the same size while the copper ones are about 2/3 of their diameter and are thinner. 

Gold: smaller and therefore worth a bit less than a Gold Crown - about 5/6 of it. On one side of the coin there's engraved the word Isshthazz - and below it there's a relief of this "fanged snake". On the other side you see a relief of the Sun with eight rays emanating from it - the Ss'tiss symbol for the God Sun.  

Silver: worth 1/8th of a golden coin. One side shows the threatening sting of a Chrezznith (Scalodon) while on the other side you find the same symbol of the God as on the golden coin. 

Copper:  worth 1/12th of a silver coin. On one side it shows a relief of a flame and on the other side the usual symbol of the Sun.

1 Chrithzesh (Gold) = 8 Thrincz (Silver) = 96 Lyssth (Copper)

There is almost no way to determine the exact age of the coins if not by how they look and how thick they are:

The older (and thinner) the coin is, the greater the chance for it being rejected by the merchants, or accepted at a fraction of its original value.

Every five years those coins too old (and thin) to be used any longer are collected and replaced. This way the government prevents an uproar of the lower classes (who are the ones owning mostly copper coins which last least of the three sorts of coins) - especially in the cities. The collected coins are brought to the mint where they are melted and the metal is used to create new coins. Due to this habit almost none of the old(est) coins do still exist. That means that the above mentioned looks of the coins are pretty much the standard.