Woohoo! I can pull up something I wrote a while back:
Our own history shows us that individual arbitrary monetary decisions can be passed down for centuries. The vast majority of European monetary systems were originally based upon the decisions of Charlemagne, who ruled that the fundamental unit of value would be a one livre de Troyes (pound of the French market city called Troy) of silver, which would be divisible into 240 denier (from the Latin denarius--a small silver coin. With some alterations. Denier becomes some variant of "penny" in northern Europe, named for a King Penning who made excellent-quality denier coins. This system survived into the 1800s in nearly all of Europe, with the British Pound, Spanish Peso, French Livre, and Italian Lira, all of which mean "pound" (even the German Mark comes from an obsolete half-pound measure called the "mark"). Likewise, Charlemagne's "base-240" division of his livre was used until the 20th century in parts of Europe (England didn't begin using decimal currency until the 1960s, for example).
Therefore, it doesn't stretch the boundaries of human nature too much to presume that a culture might retain apparently arbitrary and archaic monetary practices for more than 1000 years. Let us presume that, long ago, in the old Suloise Empire, there was a treasurer's assistant named Egar the Fastidious. Emperor Zeeckar appoints him in 4123 SD to regularize the currency as part of that Emperor's overal vitalization and "purification" program.
Egar comes to the conclusion that the best way to protect against counterfeiting is to rigidly standardize all currency. Thus, under his advice, Zeeckar issues the Currency Purification Act. From that point onwards, the only legal Suloise currency was to weight 50 per Suloise Pound (50 being the emperor's age at the time). Likewise, this currency was fixed at a diameter equal to that encompassed by Emperor Zeeckar's thumb and forefinger and of a thickness so that twenty in a stack should equal the length of the first joint of the emperor's thumb (which coincidentally makes for a coin the diameter of a US 50-cent piece and the thickness of a US dime). Egar's anti-counterfeiting method was to measure the water displacement of a measured weight of coins.
These standards would be difficult to carry out, since they required mixing the precious metals with a great deal of "base" metals (called "debasement"). Zeeckar nevertheless approved because they could be used as an excuse to mint enormous amounts of low-quality coins in order to finance his Purity Wars. While his successors attempted many times to reform this public-relations disaster, wars and debt often forced them to return to Egar's debased system. Two changes that did become popular were the addition of "half coins" (diameter of a US nickel) and "third coins" (diameter of a US dime), both of the same thickness as "whole coins". The "three-quarter coins" (diameter of a US Sacajawea dollar) and "two-third coins" (diameter of a US quarter) were also used but were less popular. Since these coins were all struck at the same thickness, their value was equal to their "size" compared to a "whole coin" of the same alleged metal content.
We now come to present-day Oerth. Even though other coins now are commonly used, the standard of exchange is still Egar's Coinage, which is still struck the mint of the City of Greyhawk and other decadent locales where one can expect a highly debased coinage. Several younger and wealthier realms have abandoned the size requirements, although for some reason the idea of 50 coins per Suloise Pound is still very popular.
In any case, enough game background, let's get to rules.
A few slight adjustments had to be made in order to get all the coins to actually fit the size and weight requirements. This changed the value of the silver coin, but in no way that is grossly complicated. I realize that I could have just hand-waved and said "It's magic." But it's the copout of an inferior mind to just pull out "it's magic" for every tricky detail. Thus, I presumed that coins would be alloyed with zinc or lead, two popular choices for making coin alloys in our own world--because they are very *CHEAP*. A "magical" solution wouldn't be cheap enough to use for coinage unless the world has every peasant carrying a +1 pocketknife and wearing enchanted clothing.
The basic Egar coinage system (using the rates of exchange of Greyhawk City) is as follows:
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Coin Purity Value (in "The Gold")
The Copper 87% 1/100
The Silver 66% 1/5
The Gold 16% 1
The Platinum 14% 10
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Greyhawk also mints half-coppers, half-silvers, third-silvers, half-golds, and half-platinums. Except for The Copper, the base metal used is zinc. Lead is used in The Copper. The very low purity of "gold" and "platinum" coinage makes them susceptable to counterfeiting with lead and zinc. The "gold" coins aren't even yellow.
As I said, other realms have been more free with this system. They have kept the 50 coins per pound principle but abandoned the unwieldy size in favor of smaller-volume, higher-purity coinage. The Fyrondy Crown is very popular, being a 95% gold coin the size of a Greyhawk "half coin" (but still 50 per pound). It is valued at 6 to the Greyhawk Gold on the street. Since this is actually slightly higher than its metal value, Greyhawk has attempted to outlaw its use within city confines to no avail.
Other coin sizes at the "50 per pound" principle are presented below for your use:
Copper Coins
Size Purity Remainder Exchange
Quarter 5% Pb 1/1750 (aka "slugs"--used
by peasants for making change)
Dollar 60% Zn 1/145
Coin 87% Pb 1/100
Silver Coins
Size Purity Remainder Exchange
Quarter 10% Pb 1/32
Dollar 36% Zn 1/9
Coin 66% Zn 1/5
Gold Coins
Size Purity Remainder Exchange
Dollar 9% Zn 9/16
Coin 16% Zn 1
Quarter 33% Zn 2 1/16
3/4 Coin 41% Zn 2 9/16
Nickel 49% Zn 3 1/16
2/3 Coin 59% Zn 3 11/16
1/2 Coin 95% Zn 5 15/16
Platinum Coins
Size Purity Remainder Exchange
Dollar 8% Zn 5 5/7
Coin 14% Zn 10
Quarter 28% Zn 20
3/4 Coin 35% Zn 25
Nickel 42% Zn 30
2/3 Coin 51% Zn 36 3/7
1/2 Coin 81% Zn 57 6/7
Penny 92% Zn 65 5/7
Quarter 8% Ag 6
3/4 Coin 18% Ag 13 1/9
Nickel 26% Ag 18 4/5
2/3 Coin 38% Ag 27 1/3
Penny 40% Au 32 1/3
Notes:
"Size" either gives the equivalent US coin size or the "Coin" size as explained above. All these coins weigh 50 per US pound. The US "dollar" coin is the Sacajawea dollar coin. "Exchange" is the nominal value in the "Greyhawk Gold" that I'm presuming is the standard D&D "gold piece". "Remainder" names to the "filler" metal used to adjust the weight. The value of silver or gold used to "fill" platinum coins has been taken into account.