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Context

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).

My Question

Is there indeed a standard gemstone size in the DMG's treasure tables, and if yes, what is it?

Parameters

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:

  1. 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
  2. Inference from real world gemstone value, applied to the RAW. This SE answer to a similar question has some helpful guidance.
  3. 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.

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    \$\begingroup\$ For those answering with a specific direction, please consider applying Good Subjective. You can read more on that in this meta. \$\endgroup\$ – NautArch Oct 17 '18 at 18:31
  • \$\begingroup\$ @XAQT78 See this FAQ for why your comment was removed. Thanks! \$\endgroup\$ – SevenSidedDie Oct 18 '18 at 1:26
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It's up to you

As this answer covers, there are a lot of levers when valuing gemstones. They include: size, clarity, cut, and color.

The value is based on all of those, so the description is entirely within your control.

DMG descriptors are descriptors only

The color descriptions of the stones in the DMG aren't equivalent to the ratings used by the Gem industry. They are just a means for you to describe. Color in terms of value is much more complicated (see the link above on levers).

Variability in description

Want a bigger stone that doesn't have as good clarity, color, or a good cut? You got it.

Want a stone that's got perfect color, clarity, cut but is smaller. You got it.

It's entirely up to you in how you want to describe your gems to your players.

Issues in subjectivity: It's opinion based.

User Slagmoth also brought up an excellent point with regard to subjectivity in valuation. I'm going to quote their comment here:

In real life valuation of such things is problematic and especially in the case of art very subjective. In a fantasy setting where you stumble across a primitive culture, the value of your "5000gp diamond" might be simply a day's meal, as it is nothing more than a "perty rock" to them. The gem and art parts of the rules always baffled me as they make little sense in any reasonable way, except in the context of a more modern culture and it assumes everyone values things equally.

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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:

\$ \begin{array}{|c|l|} \hline \text{Score} & \text{Modifier} & \text{Description} & \text{or Size} \\ \hline \text{01 -25} & \text{10 g.p. each} & \text{Ornamental Stones} & \text{Very Small} \\ \text{26-50} & \text{50 g.p. each} & \text{Semi-precious Stones} & \text{Small} \\ \text{51-70} & \text{100 g.p. each} & \text{Fancy Stones} & \text{Average} \\ \text{71-90} & \text{500 g.p. each} & \text{Fancy Stones(Precious)} & \text{Large} \\ \text{91-99} & \text{1000 g.p. each} & \text{Gem Stones} & \text{Very Large} \\ \text{00} & \text{5000 g.p. each} & \text{Gem Stones (Jewels)} & \text{Huge} \\ \hline \end{array} \$

Average Value of a gem depends upon its type, quality and weight. A huge semiprecious stone - carnelian, for example - is worth as much as an average gem stone, quality being equal. Size may vary from stone to stone, a 50 g.p. ornamental stone being of above average size, while a 50 g.p. gem stone would most likely be very small. (DMG, AD&D 1e, p. 25)

It then classified which common gemstones fell into the above categories. Two examples.
Example 1 (excerpted):

ORNAMENTAL STONES, Base Value 10 9.p.:
1. Azurite*, 2. Bonded Agate 3. Blue Quartz 4. Eye Agote 5. Hematite' 6. lapis Lazuli*: 7. Malachite* 8. Moss Agate 9. Obsidian* 10. Rhodochrosite* 11. Tiger Eye 12. Turquoise*: light blue-green

Example 2 (excerpted):

FANCY STONES, Base Value 100 to 500 g.p.:
1. Amber 2. Alexandrite (100) 3. Amethyst (100) 4. Aquamarine (500) 5. Chrysoberyl: (100) 6. Coral (100) 7. Garnet (100) (500(violet)) 8. Jade: (100) 9. Jet: (100) 10. Pearl(100) (500) 11. Peridot: (500) 12. Spinel(100) (500) 13. Topaz (500) 14. Tourmaline (100)

For the whole treatment, with tables and more detail, you need the AD&D 1e DMG. It can be purchased on line, 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):

Agate: Restful and safe sleep
Carbuncle: powers of dragon sight
Coral: cures madness, calms weather
Ruby: Gives good luck
Topaz: wards off evil spirits
(1e DMG p. 26-27)

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.

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They are exactly the size, cut, color, clarity etc. that will end with them being the listed value

This question begins with a flawed assumption:

I believe we are forced to assume a standard gemstone size for this table

The tables are not sorted by size of the gem or any property of the gem besides their monetary value as described. Almost all factors are left out of that description, which you can interpret as all other things being equal or as all other things being irrelevant.

Yes an agate the size of your head will be worth more than a sapphire as big as a grain of sand, but this isn't a table describing every gem in the world but rather a table describing common varieties of gem, as normally found, sorted by an assigned value.

So the properties of the gem can vary in any way which balances the gem out to be the exact value shown on the table.

It's all a suggestion

Don't feel as though the gems in this table are the only possible ones to reward. You can describe any value of gem as being any variety with the appropriate changes to it's properties. The tables in the DMG exist for flavour to replace simply saying "A 10 gp gem"

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  • \$\begingroup\$ "The tables are not sorted by size of the gem or any property of the gem besides their monetary value as described." That's not so. As referenced in my question, the tables do distinguish color and clarity differences for several gemstone types. How is the table in any possible way useful in describing the relative value of gemstones if their quantities could be anything? It means literally nothing to say that diamonds are worth more than agates without assuming value-per-quantity. \$\endgroup\$ – Pink Sweetener Oct 17 '18 at 18:21
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    \$\begingroup\$ @PinkSweetener Exactly. The table isn't useful in describing the relative values of different gemstones, because that isn't what it's for. The table is for assigning a gem type to an already valued but undescribed gemstone, and nothing else \$\endgroup\$ – Sir Cinnamon Oct 17 '18 at 18:22
  • \$\begingroup\$ I like this answer because it's absolutely correct: the only thing the rules define about a 5,000 gp blue diamond is that it's worth 5,000 gp. Everything else, including whether to use the table, is up to the DM. \$\endgroup\$ – Quadratic Wizard Oct 17 '18 at 23:55
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Homebrew: 1 carat per 100gp

For pure convenience, I ruled that gemstones averaged about 1 carat per 100gp (in the real world they increase exponentially, but big gems tend to be more common in fantasy anyway).

Based on the 1e DMG size tables mentioned above, that would result in:

  • Small = 100gp = 1 carat = 6.5mm = 1/4 inch
  • Average = 500gp = 5 carat = 11mm = .43 inch
  • Large = 1000gp = 10 carat = 14mm = .55 inch
  • Very Large = 5000gp = 50 carat = 23mm = 1 inch

Fancy stones: divide value by 10 (i.e. 10gp per carat)

Semi-precious stones: divide value by 20 (5gp per carat)

Ornamental stones: divide value by 100 (1gp per carat)

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    \$\begingroup\$ I actually really like this solution. It's simple and cuts through the fat. You could even add prefixes or colour variations as cost multipliers. Example: Blue 5 Carat Diamond would be 1000G because the colour adds a 2X multiplier. Same with a prefix like Rough (0.5X multiplier), Fine (1.5X multiplier,) Exquisite (2X multiplier), or Perfect (3X multipliers). \$\endgroup\$ – Lino Frank Ciaralli Oct 17 '18 at 23:46
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Homebrew: All gems in the table are princess cut, good clarity and colour and are 1cm at their widest point in every dimension

This homebrew has the following benefits:

  1. Standard scaleable size using a simple metric measurement easy to multiply and divide
  2. No less realistic than a standard price for all gems of a single type, as the DMG lists (Not all gems are grown equal)
  3. 1cm gems are approximately the size that are depicted in media and roughly the size people would assume a gem would be (A nice dramatic bag of gems rolls onto the poker table...)
  4. Mechanically changes nothing about the game (Since the gems are listed without size, and the gold value is the same we can describe it as needed)
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    \$\begingroup\$ Have you actually used this homebrew before, or is this just speculation? I realize that, regardless, it demonstrates your point that for 5th edition, it really doesn't matter, but still. The stack in general frowns upon speculative homebrew solutions. \$\endgroup\$ – Adam Oct 17 '18 at 18:38
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    \$\begingroup\$ @Adam I can't say I ever have used this homebrew, but I am willing to guarantee it doesn't unbalance the game. This question is asking for essentially flavour only homebrew and I don't know if there is precedent for how to answer that on this stack. \$\endgroup\$ – Sir Cinnamon Oct 17 '18 at 18:42
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    \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – nitsua60 Oct 18 '18 at 1:58
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I use the assumption that they mean "given a gem of the same size, these are the values". If they're not the same size, I use the table in the original AD&D DMG. Modifiers I apply are these:

Uncut = value/10: I know this seems extreme, but the yield for cut gems from rough is surprisingly low even using advanced techniques, cuts and equipment.

Clarity: Depending on the normal clarity of the stone, this can provide a modifier of valuex2 (or more) to value/10. If you're being really fancy, look at the type of stone: a flawless emerald has a higher modifier than a flawless diamond, because flawless emeralds are far rarer than flawles diamonds. A gem with inclusions or cracks is worth far less than the standard: an unflawed (but not completely flawless) gem.

Color: The less the color matches the desired color of the stone, the lower its value. A 5000gp ruby would be expected to be an unflawed pigeon's blood red. A less intense red would drop the value - if it is closer to pink than red, perhaps treat it as a sapphire instead.

Don't hesitate to modify the tables: one modifier I use is that non-crystalline stones increase in value differently from crystals - large crystals are valued as per the next table up, very large as two tables up and huge as three tables up, whereas large non-crystalline stones (such as agate and malachite) double in value, very large are x4 and huge are x8.

If your culture values the color purple, shift amethysts up a category (which is still lower than where they were during the middle ages). A highly lawful culture may consider the opal bad luck due its chaotic nature and value it lower. If gemstones have inherent magical properties, that can also shift the values.

In the end, it's up to you.

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If you just want to be able to answer a player's question, or have an arbitrary size for whatever reason (probably some specific use in an insane plan), then Here is a d4-based Roll for sizes:

1- size of a fingernail or 1/4 inch.

2- size of a thumbnail or 1/2 inch.

3- slightly broader than a copper piece 3/4 inch

4-almost an inch across.

The point of this table is to provide sizes either for some added flavour to give your players, or to determine if a gemstone would work as, I don't know, the replacement for part of a magic item-which you could also determine the likelihood of and then roll for it with the d%. I would like to add that I have used the d4 table once or twice, because many magic items in my world require powdered gemstones or precious metals in order to work.

Ultimately, the gemstones are there to provide some variety to the piles of gold coins lying around at the end of adventures, in much the same way as platinum/electrum pieces do. The size of a gemstone is incidental to that role, and few players would care deeply. As such, a gemstone is however big you want it to be.

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    \$\begingroup\$ Welcome to the site! Please consider applying Good Subjective. You can read more on that in this meta. \$\endgroup\$ – NautArch Oct 17 '18 at 18:46

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