Garnet coins

In Vamata, red and orange garnets are readily available in several large areas along the surface. Because the gem is more prevalent and requires less invasive mining than most metals it is used as the standard currency. Most garnets are harvested and cut before being used as currency, though exceptionally small garnets are often left in their uncut form.
Historically the gems were exchanged in small pouches or piles. This obviously led to some issues as there were discrepancies in the size, weight, and value of the gems based on individual assessments.When King Vili first took power he sought to stabilize trade. He did so by issuing garnet coins.
These coins, made of iron with embedded garnets are standardized so that each type of coin is worth a set amount based on the gems within it. Garnets that are too large to embed in coins are often used for high value jewelry or as a "cash in a coffee pot" type saving system. Garnets with colors that range out of the more common red/orange are understandably more valuable and are therefore less likely to be embedded into coins. Since the worth of the coins comes from the gems embedded in them the metal coins are often uneven and not perfectly round.
The coins come in three denominations; bit, shards(10 bits), and glimmers (aka glims, 10 shards).
Bits are roughly half inch across and have 2-3 very small (1/16 - 1/8 of an inch) garnets embedded in them. Shards are roughly an inch across and have 5--6 very small (1/16 - 1/8 of an inch) garnets embedded in them. Glims are roughly an inch across and have 1-3 larger but still small (1/3 - 1/2 of an inch garnets embedded in them.
Gems that are too large for shards or bits but not large enough for glims are often cut into two smaller gems and then used for shards and bits.

Rarity

All three types of coins are readily available on Vamata. Since other currency is often not accepted outside the port/capital city there are exchanges set up near the dock. Outside of Vamata the garnet coins are much more rare (though often recognized in large trade centers). The coins can be much more valuble outside of Vamata based on the condition of the embedded garnets.

Dimensions

Bits - roughly a half inch across; Shards & Glimmers - roughly an inch across