The Adrakian Empire set forth the original Soplan standardized set of coinage - the Gold, the Silver, and Copper coinage called as a set "the Triad." For the Adrakians, silvers were the coin representation of Kaygys, the second of the Dragon-Gods of the Adrakian Pantheon who was also worshipped through the mercantile arts.
The silver coin has been the general standard across Soplas for trade, with copper being used to more exactly balance the trade and gold for deals involving larger amounts of silver. Some merchants dealt in Halves, or silver coins cut in half, and Slivers, or silver coins cut into quarters, in order to avoid dealing with copper pieces.
At the fall of the Empire, the various political powers retained the familiar Triad for their international economy until it overtook attemtps at a local-only economy. Each region mints their own coin, and may accept foreign at a different exchange rate than local.
Currently, Silver fell out of use due to the events of the Gold Flood devaluing it and the other coins of the Triad. It has been replaced with brass coins since.
If a Merchant accepts being paid in any of the triad, they generally inflate their prices to cover the potential loss in case the coins turn out to be counterfeit.
Some regions break a silver from halves and slivers into nubs, or half a quarter piece. Others make a double-silver, or a coin made thicker than the average silver and valued at half a gold piece.
10 Silver = 1 Gold or 100 Copper
15 Silver = 1 Brass (Inflated price if used in a trade)
I firmly hold that gold is impractical as coinage due to density alone, but you've skirted the rarity issue nicely, for which I begrudgingly applaud you. :)
Nyehehehehehe. That being said, back when this was a D&D focused world, even D&D claimed that Silver was the primary currency to judge how much to pay hirelings and such. As for why I personally swear most everything is priced in gold pieces in D&D, I have no idea. But that's a topic for another day. As for using gold to represent many silvers, I figured that was because no one wants to track multiple coin purses full of silver when trying to make a singular large purchase. Even when the Triad was the main currency, gold itself wasn't used often. And that's my excuse for not paying my players' characters more often. xD