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.