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Preface:

I am not tagging this for a specific edition, because the subject is very narrow. I believe it has not changed much through the editions. If the reviewers find that an edition is required, I'll add one.

Background

Given the medieval / renaissance technology, I am inclined to believe that the fineness of gold would be around 18 to 22 Karats. ref1 and ref2

There is the "magic" argument, and pure gold could be created by wizards. However, all settings have gold coins (even Dark Sun), and not many of them have enough mages to make this commonplace. Maybe only Netheril, who knows.

Question

What is the fineness of gold coins in D&D? Has it been discussed, or disclosed, in any official source material?

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    \$\begingroup\$ Why can't gold have different fineness in different game worlds or settings? \$\endgroup\$ – enkryptor Oct 2 '17 at 15:42
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    \$\begingroup\$ I'm inclined to think this is a bit broad too -- what problem are you trying to solve in asking this? You're asking for every setting and every edition, and though it may only be defined in some, that just means it's more likely to just be in a setting and edition you're not using -- what has that be useful to you? \$\endgroup\$ – doppelgreener Oct 2 '17 at 15:55
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    \$\begingroup\$ @Mindwin Given answers are expected to be reasonably comprehensive, you are asking for "every" by not otherwise limiting it down. I could revise that from "too broad" to "unclear", in that you're not saying what problem you're solving because frankly this is bizarre in scope. You've acknowledged you would expect it to vary between settings, for instance. \$\endgroup\$ – doppelgreener Oct 2 '17 at 16:57
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    \$\begingroup\$ It does look like an XY problem, yes. We would ask at times like this: what problem are you trying to solve? We do prefer people be specific to their own issues, rather than artificially broaden their solution to potentially help others, so a desire to create a broad knowledge base is a laudable goal but not what we look for in questions -- ironically being a bit selfish is the best way to be helpful to the most people, since trying to make questions broader tends to backfire in making them less helpful to any specific person. (Closed as unclear, per your request.) \$\endgroup\$ – doppelgreener Oct 2 '17 at 16:59
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    \$\begingroup\$ The main reason I think edition matters is that gold coins were 10 to the pound in earlier editions and then 50 to the pound later. \$\endgroup\$ – mxyzplk Oct 3 '17 at 1:26
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As the rulebooks do not give an actual answer to this, the best I can give you is an approximation.

What we know:

Throughout D&D rules across editions, a gold coin has most frequently been described as being about the same size as a US Half Dollar Coin. This citation comes from either a verbal description ('about an inch across.' ref: p212 of 4E PHB), or from an 'exact size' picture (ref: p168 on 3.5E PHB). So, for simplicity's sake, I am going to use the thickness of a half-dollar coin (is thick enough to be sturdy) and the 'inch across' quotation and image.

A quick check on Wikipedia tells me that a Half Dollar Coin is 0.085 inches thick. This gives us a volume of 0.067 cubic inches.

Given that pure gold weighs 0.7lbs per cubic inch, we can compute that a half-dollar sized coin should weigh 0.0469 lbs.

In Basic D&D and AD&D 1E, a gold coin was described as weighing a tenth of a pound. Which is...frankly...too heavy. The coin would have to be nearly twice as thick in order to give you that much heft.

However, from AD&D 2E onward, the weight of a gold coin has consistently been held to be one third of an ounce, or 0.0208lbs. (ref: 5E PHB p143, 4E PHB p212, 3.5E PHB p112). Thus, we could reasonably assume that either the coins are smaller than advertised, or they are not pure gold.

Historically, copper has been used as a common material to alloy gold with for making coinage. Copper weighs 0.324lbs per cubic inch, which would give us a pure copper coin weighing 0.0217 lbs....which is still too heavy. So it can't be a copper alloy. Tin is another option, and it weighs 0.204 lbs per cubic inch. A pure tin coin of this size would give us a mass of 0.0137 lbs. Tin is the lightest material that gold is commonly alloyed with that would be available in the middle ages, so we can guess that the gold is alloyed with tin.

So, to compute this, we have two equations as a system...given that x is the percentage of gold and y is the percentage of tin.

$$x + y =1$$

Representing that the percentages must add up to 100%

$$x*0.0469 + y*0.0137 = 0.0208$$

Representing that the percent of gold and tin must add up to 1/3 of an ounce.

Solving this system of equations gives us the following values

$$x = .2139$$ $$y = .7861$$

Which translates to a coin that is 21.39% gold and 78.61% tin.

This translates to 5K Gold...a quite low gold content. With a gold/tin mix, I'm not sure this would even still look like gold. If the coin were smaller, thinner, or heavier...it could have a higher gold content but, as described in the PHBs...5K is the highest fineness I can mathematically produce..

Note

There's a complication to this worth mentioning. According to 5E PHB p157 and 3.5E PHB p112: 1lb of gold is worth 50gp. And weighing a third of an ounce, 50 gold pieces equals one pound. This equality seems to exact to be a coincidence...that a pound of gold coins is equal in worth to a pound of gold? While not any form of concrete evidence...it does raise a secondary possibility:

It is distinctly possible that the coins are, in fact, 'pure gold,' and the mismatch in weight and/or size is simply because the designers didn't bother with the math, and set the weight based on a gameplay decision rather than on realism. Simply saying "this seems like a good weight for a coin, in terms of how much weight an adventurer can carry...and we want our coins to be pretty large in size...so make them about the size of a half-dollar."

And if you're concerned about difficulty....D&D is set in a society that has reliable and sturdy steel production. Refining 'pure gold' is easier, and was accomplished far earlier in history.

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    \$\begingroup\$ I have no idea. My rig went offline for a few minutes while I was typing the answer...but otherwise....I guess I'm magic? (Or I just discovered a bug in the stack software...which would be weird.) \$\endgroup\$ – guildsbounty Oct 2 '17 at 17:33
  • \$\begingroup\$ I checked my timings, it appears that the question was closed while I was without a network connection. Bug report submitted on the core stack exchange meta. \$\endgroup\$ – guildsbounty Oct 2 '17 at 17:44
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    \$\begingroup\$ There's a small leniency window after a question's closed in which clients are still allowed to post an answer, if they somehow didn't get the memo that the question had been closed. \$\endgroup\$ – doppelgreener Oct 2 '17 at 17:58
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    \$\begingroup\$ coins were worth more than the metal they were printed on from the time of the invention of coinage in ancient Mesopotamia through the fall of Rome. After the fall of Rome, Roman currency continued to be used for pricing all across Europe, even long after all the actual Roman currency was locked up in rich people's vaults. People would list prices in silver denarii and bronze asses, but would actually sell/trade goods either for other goods with commonly agreed upon values (like D&D's trade goods, but for different goods) or for newer, often locally-minted currencies like the pfennig. \$\endgroup\$ – Please stop being evil Oct 2 '17 at 21:27
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    \$\begingroup\$ In PHB 3.5 the coin image has a shadow. I wonder if anyone can calculate its thickness from the shape of that shadow. \$\endgroup\$ – Ols Oct 2 '17 at 22:13

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