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The material requirement for the spell chromatic orb is a diamond worth at least 50 gp, but in the Dungeon Master's Guide a diamond is listed under the 500 gp gem list. How small would a diamond be that is only worth 50 gp?

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Not necessarily any smaller than the 500 GP diamond

In the real world, diamond value has as much to do with cut, clarity, and color as it does about size. So the price difference between a "regular" 500 GP diamond from the DM Guide and this 50 GP material component wouldn't necessarily denote a size difference at all.

(Maybe jewelers in the Forgotten Realms have bargain bins of ugly diamonds, which they get out when mages come to shop.)

Just a reference, a standard one-carat diamond is 200 mg and 6.5 mm in diameter. Compare this to a 9 gram coin - gems in general are pretty small.

Word from the expert

I consulted Diana Bogue, a multiclass gemologist / antiquarian (GIA AJP, Diamonds Graduate) about diamond prices in the past. She had this to say:

Diamond cutting was much more rudimentary...only a diamond could cut a diamond properly back then (they didn't have lasers), and there wasn't much in the way of cut design work.

A cloudy, inclusion-filled non-jewelry quality diamond is much cheaper than a smaller clear diamond.

At my local antique shop, there's a black and cloudy white, chipped diamond that's about one carat that is selling for 50 bucks. On the other hand, a 10 point clear diamond is over one hundred.

If you can buy a one carat diamond for US$50 now, I think 50 GP would also do it.

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    \$\begingroup\$ IIRC, some spells call for ground-up gems. How big is 500gp worth of diamond dust? \$\endgroup\$ – acbabis Jul 22 '16 at 19:10
  • \$\begingroup\$ I wonder how small a gold coin would be today if it were worth $1 in D&D5e gold. And how much a D&D 5e gold piece's weight (1/50 pound) is worth today. Let's see, that's $1,324.00 per ounce x 16 ounces/pound, divided by 50 = $423.68. \$\endgroup\$ – Dronz Jul 23 '16 at 2:19
  • \$\begingroup\$ @Mason: OTOH you have Milo Amastacia-Liadon's observation that salt has a fixed price explicitly listed in the player's handbook , and breaks the magic crafting monetary requirements by buying up tons (literally) of salt dirt cheap. \$\endgroup\$ – Hurkyl Jul 23 '16 at 9:45
  • \$\begingroup\$ @Hurkyl In 3.5e, the flesh to salt spell was one of the more notorious money exploits, since it could easily produce hundreds or even thousands of pounds of salt in a casting. \$\endgroup\$ – KRyan Dec 27 '18 at 15:52
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Let's take a pull in mathematics, economy and physics.

First, let's look up the weight of a gold piece. Google even answers that question:

In “Basic” D&D (and previous editions) and First Edition AD&D, despite the described weight, gold pieces are considered to weigh a tenth of a pound (1.6 standard ounces) each for encumbrance purposes, with 10 gp weighing one pound.

However dungeons.wikia says:

Starting in Second Edition AD&D and continuing through Third and Fourth Editions, gold pieces are considered to weigh approximately a third of a standard ounce (9 grams) each, which equal about fifty gp to a pound, while maintaining the size equal.

Well, that is disappointing, but not unbearable: the later D&D coins just are worth 1/5th of the earlier, so let's roll with the old coins first.

Now, 1.6 standard ounces of gold for the old D&D standard... what kind of diamond you would get for that? If gold is measured in standard ounces, then it is 22 karat gold, so let's convert to fine ounces, aka troy ounces: 1.45 fine ounces of 24 karat gold.

Now, Gold price nowadays fluctuates just like diamond prices, but for a rough estimate, the gold price has hung around 400 USD/oz (troy) from the 80s-200s, and is around 1250 USD/oz (troy) since about 2012. So... for calculations sake and to cut out inflation, let's guesstimate that the actual 'worth' of a gold piece if it would be struck in this world would come up to, let's say 1000 USD for first edition coins. Or, if we use the new coins: 200 USD.

Now, we have a 50gp diamond. What kind of diamond you get for 50 grand/10 grand? Now, I found a pretty nifty site that tells about diamond prices. The relevant snippet for my estimations is here:

$10,000 – $20,000 This is the lowest price range where you can reasonably expect to find a good selection of quality 1.5 carat stones (and the 1-carat stones in this price range will stir the heart of the toughest critic [=excellent quality]).

$30,000 – $50,000 - In this price range, you can expect to find excellent quality stones up to 2.50 carats.

$50,000 – $100,000 In this price range, you can expect to find excellent quality stones up to 4.0 carats.

So, assuming the 1000 USD/gp, the 50gp nets us a pretty nice, clear and almost perfect 2.5 carat stone, the 200 USD/gp gives us a shiny stone of a bit less than a carat of those traits. With the 100 gp we get an equal clear stone that is more like 4 carats or for the newer gold coin factor, a 1 carat stone. That is assuming excellent quality stones. Now, how large are 2.5 carats and 4 carats? They only weigh half a gramm and 8/10th of a gram (0.017 and 0.028 standard ounces for imperials), but that is a weight and weight not translate to radius and diameter well, as we all know:

  • \$V=d \times \pi \times r^2 \times f\$

    \$d\$ is the depth of the diamond, \$r\$ the radius (=\$\frac{\text{diameter}}{2}\$), \$f\$ is the formfactor and is derived from the actual shape: it is \$1\$ for a cylinder, 1/3 for a cone.

  • \$M=V \times \text{[density]}\$

Most diamond distributors don't list dimensions, but I was lucky to find a carat-size chart: 1 carat stones have a diameter of 6.5 mm, 2.5 carat are roughly 8.8 mm and 4 carat stones tend to be 10.4 mm in diameter. This however is assuming the afforementioned perfect and excellent stones. Still, it shows quite nicely, that diamonds of larger value are not nessecarily that much larger: while size (and therefore weight) does play an important factor, the other factors (Color, Cut, Clarity) do also have a huge impact on the value.

Still, both diamonds in question are of a neglectable size in comparison to almost all other items. In comparison between the two (jewlery grade) stones it all boils down to:

100 gp carat stones do either have clarity, cut or color of better quality, or they are visibly larger than the 50 carat variant.


Just because I was asked about it, an addition:

Up there I only calculated with the prices of Jewlery grade diamonds, taking a guesstimated price for a gold coin. Now, Maybe a comparison might be more worth it:

Let's take a fix-point of 1000 USD/gp, and consult the table again.

  • 7.5 gp = 7,500 USD = 0.8 carat jewlery diamond
  • 10 gp = 10,000 USD = 1.0 carat jewlery diamond
  • 20 gp = 20,000 USD = 1.5 carat jewlery diamond
  • 30 gp = 30,000 USD = 2.1 carat jewlery diamond
  • 50 gp = 50,000 USD = 2.5 carat jewlery diamond
  • 100 gp = 100,000 USD = 4 carat jewlery diamond

Plotting those in excel and putting a trend line into it that has to cut (0,0), the function for carat/gp I get is:

  • \$\text{weight in carat}=0.0454 \times \text{price in gp}\$

Playing that game agian for 200 USD/gp, the formula is:

  • \$\text{weight in carat}=0.0119 \times \text{price in gp}\$

Now, this still estimates jewlery grade diamonds. However, there are also industrial grade diamonds, that price at 0.3 to 10 USD/carat with large stones in the 200 USD/carat range, depending on grain size; synthetic diamonds range (depending on stability) from 0.4 to 4.50 USC/carat usually with large stones in the hundreds again. But taking huge natural industrial diamonds does not work because at some point their price jumps back to the jewlery grade prices just for sheer size. And the formula above does not work for named or huge jewlery gems either, as those tend to be priced for their story or previous owners to a large degree. Still, by comparison and using grain size as an indicator, the factor in the trendline above would be something like factor 10 (42) larger for industrial diamonds. So, by rule of thumb:

  • If you can't use a diamond for jewlery, multiply its weight by 10 ("expensive coins" or low disparity between the price for "magic grade" and "jewlery grade" diamonds) to 42 ("cheap coins" or a high disparity) to gain the same value.
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    \$\begingroup\$ +1 for the very useful chart and detailed answer. Would it be worth reminding that diamonds fall in two classes ? Jewelry diamonds have no use (in real world, this make their economics very special, ask De Beers how they manage stock, kill the offer by making diamond so emotional, or created the whole diamond wedding rings market in japan), but industrial/magic-user diamonds have and do not need the same quality, are more common, and cheaper. \$\endgroup\$ – Uriel Jul 22 '16 at 18:43
  • \$\begingroup\$ Your calculation would be even more awesome if you had managed to find the approximate relative prices of gold and diamonds, and, as @Uriel mentioned, industrial diamonds, such as those used in cutting glass, rather than those used in jewelry. \$\endgroup\$ – Baskakov_Dmitriy Aug 5 '16 at 9:38
  • \$\begingroup\$ @Baskakov_Dmitriy Did the math... actually in that low range the trend is pretty much doable as a linear progression, even if an exponetntial function can model it even better (but base 0.2419x with exponent 0.6105 is not handleable, and neither is base 0.0536x with exponent 0.7181). It's for rough estimates anyway. \$\endgroup\$ – Trish Aug 9 '16 at 7:56
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Quick Answer

This diamond. Its weight is negligible (in quantities short of a truckload), and its size is on the order of a sweet pea.

Addendum

I was thinking about this again. It's perhaps shortsighted to say that a the same diamond in D&D is worth the same to two different characters (or conversely, that 50GP gets you the same diamond regardless of where you are in the realm).

For example, a farmer with no means of liquidation might find it worthless and prefer to spend his precious gold on seeds, tools, and beer. A noble might collect them for his wife and give you a "fair" 50GP for it. A spell caster far from civilization that needs it to save the world might give you anything you ask for.

To me, this is part of the fun of being DM. I think you might want to consider where the diamond is, who is holding it, what's around, and go as deep into the rabbit hole as you want. I like to add cost to more "high-tech" items when far from cities, and remove value from goods being sold to NPCs that might not see its worth or have no use for it. Just some additional food for thought.

This was a great question by the way!

Dimensions

  • Measurements: 7.54 × 6.67 × 4.57 mm [230 mm3]
  • Weight: 2.00 carat
  • Mass: 400 mg (0.0141096 oz)

Explained

There are many variables in appraising gemstones. Size isn't really considered, being a derivative of weight. Weight is measured in carats (1 carat = 0.2 g).

Other variables may include:

  • weight
  • quality
  • color
  • location
  • history/lore (Once belonged to a famous person... Ooooooo!)

For the purposes of gameplay, I'm assuming rough estimations rooted in current RL prices are acceptable.

Let:

Based on this chart, we can infer a size of 2 +/- 0.25 carats. The quality and color in inversely proportional to the weight; e.g. as the weight goes down, you can go up a notch in quality or color.

So, a single stone worth 50gp might looking a lot like the diamond linked above. 2.00 Carat, VS2 Quality, and D color. According to our chart, this diamond is valued at $21,390... pretty close!

For reference, a 500gp diamond would be 10 carat, D color, round cut, and internally flawless.

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  • \$\begingroup\$ Thanks for the edits, but I think the dimensions table looked better as a fixed width code block. No hard feelings, though. ^-^ \$\endgroup\$ – CaptainMarvel Jul 22 '16 at 19:35
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    \$\begingroup\$ Interesting theory, but it relies on the modern valuation of Gold. Today it's about $1300/oz, 5 years ago almost $1700/oz. In 1980 it hit nearly $2000/oz, and between the two, it dropped to barely $350/oz in 2001. So we are forced to ask which of these is the correct currency conversion (if any)? You could easily argue $850/oz or $575/oz too (rough prices when OD&D and AD&D 1E were released). Functionally, there is no practical method of converting D&D currency to a real world equivalent. \$\endgroup\$ – tzxAzrael Jul 23 '16 at 16:21
  • \$\begingroup\$ You are correct, but I suppose I had the pre-20th century gold standard in mind. Markets fluctuate now more than ever. I was mostly just offering a way to think about it. \$\endgroup\$ – CaptainMarvel Aug 16 '16 at 19:25
  • \$\begingroup\$ Yea, all I was trying to say was that at some point, you simply have to assign an arbitrary conversion value, and based on that, different tables may end up with wildly different values. \$\endgroup\$ – tzxAzrael Aug 18 '16 at 14:51
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The simplest answer:

A 50gp diamond is exactly as large as a 50gp diamond.

Not terribly helpful, I'm aware, but unfortunately I've never seen any of the rules editions get into specifics about exactly how big or small a particular size of gem is.

In addition to all the variables that affect the value of real world gems (Color, Cut, Clarity, Carat, what kind of mood the jeweler's wife was in that morning, etc..), there are also all the details of the fantasy world that affect prices. Rarity, demand, other uses for the gem besides jewelry, access to the Elemental Plane of Earth, etc.

On top of that, the distinction between what we consider "precious" and "semi-precious" gems is not really made. All gems are potentially equally valuable.

Functionally, gems are simply a high-density monetary unit, useful for transporting great sums of money without dealing with the weight of dozens of chests of coins. The only practical measure of a gem's size is its value in GP.

Aside from that, the gem is exactly as big as the DM says it is.

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This question depends on many variables, the most prominent being the 4 C's: Color, Cut, Clarity, and Carat. Carat corresponds back to the size, so that is irrelevant. Uncut diamonds with large defects that make even smaller diamonds flawed could fetch a lower price than much smaller diamonds. Diamonds with yellow in them can be worth far less than clear diamonds, while blue or pink are much more valuable and reds could fetch great prices due to rarity.

Running off Timster's idea of the bargain bins at apothecaries, the value of diamonds to PCs could vary greatly depending on what qualities give the base value of the diamond. Due to lower weight, PCs would likely prefer carefully cut, crystal-clear, and courageous crimson diamonds that are really tiny. Makes it much more compact to keep revival components handy. Now, if they needed to make change for a variety of different spells (Revivify), they might prefer more uniform diamonds that can be easily sorted off when someone needs to be revived.

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Giving a system-agnostic answer here.

In fantasy, it's common to want gems to be "larger than life". However, one expects larger gems to be more valuable, ceteris paribus. I.e., other factors may matter, but we can hold them constant for our purposes.

Empirically, I recall looking at a chart of diamond prices once and finding that they vary as roughly the 4/3 power of mass - therefore, as the 4/3 power of volume, or 4th power of width/height/length (assuming uniform scaling).

Therefore, a 50gp gem is about \$0.1^{0.25}\$ (which conveniently is about 9/16) as big across as the DMG says the 500gp gem is.

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    \$\begingroup\$ your formula ignores the formfactor of cut gems and that these gain most of their later value by cutting. It only holds true for uncut and roughly spherical gems. These generally are accounted for much less than the cut gems - to make 1 carat cut diamonds you need an uncut stone that is about 3.3 carat, as you have a loss ratio of up to 70% of the original. This means, uncut gems are worth much less than a quarter of cut gems. \$\endgroup\$ – Trish Jul 22 '16 at 12:24
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As large as two pinches, or as a modest diamond ring, while some very old rules say ¼ carat

By the 5e core rules, diamonds exist of various value.

Nolzur's Marvelous Pigments (DMG p.183) can't create anything worth more than 25 gp, including the example of a diamond, suggesting that even the least valuable diamonds are worth more than 25 gp.

It is implied that whole diamonds may exist as small as 50 gp (chromatic orb, PHB p.221), with other diamonds described as at least 500 gp (raise dead, PHB p.270), at least 1,000 gp (clone, PHB p.222; resurrection, PHB p.272), at least 5,000 gp (gate, PHB p.244), while true resurrection (PHB p.284) allows the 25,000 gp cost to be paid in multiple diamonds, suggesting that diamonds this valuable are rare.

The spell revivify (PHB p.272) cites a diamond worth precisely 300 gp, suggesting that diamonds worth less than the DMG treasure list's 5,000 gp actually exist.

However, the closest direct measurement of size is given by the spell nondetection (PHB p.263), which requires a pinch of diamond dust worth 25 gp. If we can assume that this is about the same mass as a diamond worth 25 gp, rather than dust having its own market value (e.g. flawed diamonds laboriously ground into powder for spell components), then a diamond the size of two pinches of salt is worth 50 gp.

The Helm of Brilliance (DMG p.173) is described as being inlaid with 1d10 diamonds, amongst other gems. We can at least infer here that diamonds exist small enough that up to ten can be inlaid on a helmet which already has around fifty other gems inlaid in it.

Now, if we're willing to consider lore from earlier editions of the game, Dragon #8 (July 1977) ran an article titled A Re-Evaluation of Gems & Jewelry in D&D, by Rob Kuntz. According to this article, which provides a more detailed breakdown than the standard rules at the time, a diamond is typically worth 200 gp per carat, with 1 or 2 carat gems being most common, and the largest having 1,000 carats or 200,000 gp.

According to that article, a 50 gp diamond weighs one quarter of a carat. This is small enough to be humbly inlaid in a ring, and this would be a convenient way for a wizard to carry it.

All these guidelines suggest that a 50 gp diamond is about as small as a diamond can reasonably be. Any much smaller and it would be too difficult to work or facet, and it would be hard to see or handle, which defeats the purpose of a diamond as a jewel or item of value for trade. A quick search suggests that while smaller ⅛ carat diamond rings exist in the real world, smaller one-sixteenth carat diamonds are uncommon.

This roughly matches the interpretation above, which is that one-eighth carat diamonds worth 25 gp are typically the smallest found (as per Nolzur), so I'm comfortable with the assertion that, based on both current and historical lore, a 50 gp diamond weighs around one quarter of a carat and at that size would need to be inlaid in a ring to be handled easily.

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