D&D has always stipulated that 50 coins equate to 1 pound. But mithral/adamantine do have different wieghts, so we can assume that 50 adam. coins weighs much more than 50 copper coins.
So the whole thing is flawed.
No, it has not. In fact, 3E is the first place I recall seeing 50 cn = 1 lb.
Holmes (page 9) gives a 300 coins to 30 pounds as the limit of a bag.
Moldvay/Cook BX used 10 cn = 1 lb. (B63).
Mentzer BECMI uses 1 pound = 10 cn. (Basic, P 61)
Alston's Cyclopedia also uses 10 cn = 1 lb. (p. 226)
AD&D 1E specifies 10 cn to 1 pound as well (PHB 9)
I can't find the reference for coin weights in the Core Rules 2.0 PHB RTF...
And Holmes, Moldvay, Mentzer, Alston, and Gygax all had all coins (for convenience) measured at 10 to the pound.
3.0 and later used a much more realistic (but still heavy) 50 to the pound. The Roman denarius was 240 to the troy-pound, or about 291 to the US pound (643 to the kg)... as were early medieval silver pennies (typically of 90% silver, 10% copper and/or tin for durability).
Edit: For comparison US coins per pound, rounded nearest integer
Penny 181, Nickel 91, dime 200, Quarter 80, half dollar 40, dollar 56.
And UK coins: penny: 127, two pence 64, 5d. 140, 10d 70, 20d 91, 50d 56, 1£ 48, 2£ 38, 5£ 16
The €2 coin is about 53 per pound. The €1 is about 60.5 per pound, and the €0.01 is about 197 per pound.
http://www.usmint.gov/about_the_mint/?action=coin_specifications
http://www.royalmint.com/discover/uk-coins/coin-design-and-specifications
http://europa.eu/legislation_summar...roducing_euro_practical_aspects/l25028_en.htm
Now, as to Mithral... I've assumed it's just another name for platinum myself...
Silver in color: check.
Harder than iron: check
nearly impossible to work with medieval tools: check
not meltable with a typical blacksmith's forge: check
Worth more than gold: Check (tho' only a few dollars an ounce more recently)
Tho' I'm tempted to make it 10x as valuable as platinum, just for S&G's...