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In Advanced Dungeons & Dragons 1st edition there is a plethora of information on the value of gems, and their purported magical uses. But the gems are only ever listed by value, not size or weight.

How big are they? How many can you carry in a bag? When do you have to get a packhorse to haul your semi-precious stones?

Obviously the answer to this varies dramatically by the type of stone. A 5000gp carnelian will be much larger than a 5000gp diamond.

Are there any guides for gem size and weight, especially for the semi-precious stones?

(Edits welcomed on the question title)

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4 Answers

up vote 12 down vote accepted

I would call it at a 100 gems per pound unless you specifically state is a certain size (like the eyes of the Player Handbook Idol). Gems are weighed in carats and the problem is that the value per carat varies between gem types. Harnmaster is the only Fantasy RPG I know of that gives that information. Even armed with such information I feel it strays over the line of too realistic of a rule. Harnmaster rates each gem type with a price factor and then multiplies it by the weight in carats squared. Note that a carat is 2 grams. 227 carats in a pound (I rounded up).

A 100 per pound is a nice average that reflect gem's compact value at a somewhat accurate weight without having you and your players go through a lot of bookeeping. For those 1,000gp, 5,000 gp gems just make up a weight from 1/4 to 1 pound.

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I like this answer. It's nonspecific and yet very useful. Plus, it's easy to remember. –  John Fiala Dec 10 '10 at 16:40
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The record cut diamond is 105 carats, and is a couple inches long, by about an inch wide, and about 1/3" deep... Records for cut gems in Beryl and Corundum run to about 200-300 carats... That covers most of the precious stones.

Therefore, figure most huge diamonds should be under half a cubic inch; a typical sack can hold thousands. A 2' by 4' sack has a full circumference of 4', and can hold roughly a sphere of .63' radius... roughly a cubic foot... or some 1730 cubic inches.

Huge sapphires, emeralds and rubies, (and several others) run to a couple cubic inches.

A typical sized gem, however, is under 1/16th cubic inch for precious, and under 1 cubic inch for semi-precious.

For simplicity, I'd put the following benchmarks:

Diamonds: Huge 1 - 2 ci
VL: 1/2 - 1 ci
L: 1/4 - 1/2 ci
M: 1/8 - 1/4 ci
S: 1/16 - 1/8 ci
VS: 1/32 - 1/16 ci

Beryl, Aquamarine, Emeralds, Saphires, Rubies, corundum, should run about the same until VL and huge, which should be 1/2- 3/2, and 3/2-3 ci, respectively.

Most semi-precious stones should be fully a size larger than diamonds of comparable relative size...

If you want a truly epic fantasy feel, move the volumes up a step... making huge diamonds some 2-4 ci...

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If you're gonna use science, you gotta consider the space between the stones, too. This is not an easy problem. –  Adam Dray Nov 23 '10 at 16:35
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WHich is why I based upon bounding box, not fluid displacement. Packing "loss" then turns into packing gains. ——— it really IS a simple problem, Adam, provided one restricts to 1st order approximations (bounding box, spherical volume for sack contents, etc). It's accurate enough except for the A-R. –  aramis Nov 23 '10 at 19:40
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OK, while I have practically zero interest in the question, this answer and comment thread is one of the most interesting things on this site! :) –  gomad Nov 23 '10 at 23:58
    
+1. Not as simple and easily applied as it could be, but a very good sanity check for evaluating a DM's rule of thumb. –  SevenSidedDie Jun 9 '12 at 18:54
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How about one gem = 1 coin?

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+1 for simplicity. –  Jon Hopkins Jan 27 '11 at 16:56
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+1, since this roughly accords in outcome with the mathier answer of aramis and the pragmatic-gameability answer of RS Conley. –  SevenSidedDie Jun 9 '12 at 18:52
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A simple method would be to set how many gems of a given size (very small, small, average, large vary large and huge) fit in a pound. You could easily do this by looking at gem sizes in a jewelry store and then using rocks of the same size. Once you know how many of size fit in a pound then all you have to figure out value of a gem type by size.

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