The DMG treasure tables (DMG p134) describe six categories of gemstone: 10gp, 50gp, 100gp, 500gp, 1000gp, and 5000gp. What does this mean? An agate can be worth anywhere between 10gp and 5000gp depending on how big it is, not to mention other factors. Therefor, I believe we are forced to assume a standard gemstone size for this table. For the table to be of any use as a resource, a 10gp agate must be roughly the same size as a 5000gp black sapphire. The statements “an amethyst is worth 100gp” and “a topax is worth 500gp” simply do not mean anything without a size reference.
I understand that gemstone value is not entirely, or even mostly dependent on size: color, clarity, and cut are also important. But this is not any help, and only makes the existence of a table ranking of gemstone value even more baffling unless we can also assume a standard level of quality, and indeed this assumption is supported by the fact that the table already lists different grades of certain gems. Sapphires and rubies, for example, appear in different categories of value depending on their color and clarity.
However there is some slight evidence to suggest that there is no standard size. The 5,000gp gemstone list includes both a diamond and a ruby, which are often not the same value, so perhaps the ruby on this list is larger than the diamond? Even if this were the case, as I have argued above the table does not make sense without at least some reference to size, so the size variation must be slight, or within a range. Alternatively, there is a standard gemstone size and either (A) rubies and diamonds are worth the same in this world or (B) the listed diamond is lower grade or the ruby is higher grade (and indeed, there are lower grade rubies listed elsewhere in the table).
Is there indeed a standard gemstone size in the DMG's treasure tables, and if yes, what is it?
I do not require a strictly RAW answer to this question. There does not seem to be guidance on this issue within the official rules, but I can think of three possible ways of answering this question, in order of authority:
- Inference from official adventure modules that contain a
description of a gemstone’s size and value. To get at the pure
value of the gemstone as a material, such a reference should not be
to gemstones that are part of an art object
- Inference from real world gemstone value, applied to the RAW.
This SE answer to a similar question has some helpful guidance.
- If nothing else, a secondary source with more usable gemstone
tables would be helpful. This includes previous editions,
thoughtfully written homebrews, and third party publications.
All I want is a number that is plausible, based on either real life or existing D&D materials.
From a previous edition: AD&D 1e, DMG
Since you are open to previous edition material:
On pages 25-27 of the AD&D 1e DMG, an extended discussion of gem stones almost gets you what you want. I say almost because for size, it uses small, medium, large, etcetera, rather than units of measure we are accustomed to in the real world, like carat or point, or even cm/inch.
The DMG assumed that the DM was generating gems randomly, so:
It then classified which common gemstones fell into the above categories. Two examples.
Example 1 (excerpted):
Example 2 (excerpted):
For the whole treatment, with tables and more detail, you need the AD&D 1e DMG. It can be purchased online, or you can seek out a nearby grognard and borrow theirs. I can't recreate the whole set of tables without running afoul of fair use rules that SE sites must comply with.
The other neat feature of the AD&D 1e DMG breakdown of gems is the magical properties of gems that are suggested, such as (to show a few examples):
I found those suggestions to be at least as helpful as the value/color/size from the tables in the DMG, when describing some of the magical items that the players found.
Follow up for AD&D 2e: from the DMG, I find that they used the same table, roughly, but provided even less detail. Tables 85 and 86 in the DMG.
Follow up for Dragon Magazine #8 (July 1977): Rob Kuntz created an extended series of roll up tables on pages 22 and 23 of Dragon Magazine issue 8 that went into far more detail than the AD&D 1e DMG ever did. Again, I can't reproduce that without running afoul of fair use.