I’ve talked before about the physical properties of coins and other objects made of precious metals. Now it’s time to discuss gems.

Gem Categories

I use six categories of gems. Each category contains five gems (although some of these, such as “Common Quartz” actually represents a family of gems). Note that the distribution of gems is my own and is not intended to mimic the real world relative worth of these stones.

This is a list of the gems used in my campaign and the categories to which they are assigned.

Common Stones:Agate, Amber, Bloodstone, Common Quartz, Moonstone
Ornamental Stones:Carnelian, Garnet, Lapis Lazuli, Malachite, Onyx
Fancy Stones:Citrine, Jade, Opal, Topaz, Turquoise
Precious Stones:Amethyst, Black Opal, Fire Opal, Jacinth, Pearl
Gem Stones:Black Pearl, Diamond, Emerald, Ruby, Sapphire
Jewels:Black Sapphire, Blue Diamond, Star Emerald, Star Ruby, Star Sapphire

Gem Size

The size of an average gem, regardless of type, is 1″ in diameter. (Again, this is not intended to mimic the real world). There are four additional size categories, two larger and two smaller, with a doubling of diameter between each size. I want the weight and value of gems to change by a factor of ten between size categories (if an average size gem is worth 10 gold and weighs 1/20 of a pound, then a gem of the same type that is one size larger should be worth 100 gold and weigh 1/2 pound).

However, if the diameter changes by a factor of two then the volume (and therefore weight) will change by a factor of eight. This would create more complicated record keeping; I want the weight to change by a factor of ten. To accomplish this, the diameter has to change by an amount equal to the cube root of ten (roughly 2.15). For all calculations, I will use that value. But for simplicity sake I will refer to diameters of gems as changing by a factor of two between sizes.

All that sounds far more complicated than it is. Hopefully this chart will make it more clear.

TinySmallAverageLargeHuge
Diameter0.25″0.5″1″2″4″
(True Diameter)0.2154″0.4641″1″2.1544″4.6415″
Weight (pounds)0.00050.0050.050.55
Gems per Pound2,0002002020.2
Carats1101001,00010,000
 
Common Stones*1 cp1 sp110
Ornamental Stones1 cp1 sp110100
Fancy Stones1 sp1101001,000
Precious Stones1101001,00010,000
Gem Stones101001,00010,000100,000**
Jewels1001,00010,000100,000**1,000,000**

* Tiny Common Stones are not used.
** Gems with a value over 10,000 gp are merely theoretical (or at best unobtainable).

These Sizes Are Ridiculous!

I prefer the term “Fantastic”. This is, after all, a fantasy game. Typical gems (diamonds anyway) are extremely small. A wedding ring commonly has a ¼ carat diamond which is considerably smaller than the “Tiny” stones listed above. But how exciting is it for adventurers to have to pull out a magnifying glass to examine their treasure? Many books, movies, and fantasy illustrations incorporate large stones (the size of your thumb) and huge stones (the size of your fist, often exemplified as the stones that thieves pry from the eyes of an idle). Such stones are far more exciting and satisfying to players.

Obviously, a fist-sized diamond would be worth extraordinary sums in the real world. In order to incorporate them into the fantasy world, the value of these stones must be adjusted accordingly. The 1e DMG lists a base value for gems between 10g and 5,000g (with adjustments to these base values going as high as 1,000,000g). I found the method of adjusting base values too bulky so I abandoned it. However, I adjusted the base values (and added size categories) to allow for a wider range between the upper and lower thresholds. That way there can be inexpensive ornamental gems on low-end jewelry as well as legendary gems (worth 100,000g or more) that could be the goal of a great many gaming sessions.

Although these sizes are intended to be fantastic in nature, note that they are not without real world counterparts (to some degree). There is a class of diamond known as paragon that is bestowed upon flawless diamonds in excess of 99 carats. My “Average” gems are 100 carats. Clearly, 100 carats is by no means “average” in the real world but the existence of paragon diamonds does somewhat validate the sizes I’m using. The largest cut diamond was actually over five times that large (over 500 carats). The largest pearl ever found (the Lao Tzu pearl) measures 24 centimeters and weighs in excess of 14 pounds! (well over 200,000 carats).

I’m not trying to say that the sizes I am using are at all realistic but there have been real world exceptional stones that do make them somewhat more acceptable. What it all boils down to is, I wanted fantastic gems that were more exciting. It would be quite easy to simply adjust the sizes down and keep the prices the same. After all, what does it really matter if a particular gem is 1″ in diameter or 1mm in diameter? Players just want to know what it is worth.

Gem Density

Finally we come to what prompted this whole article. We know how heavy coins are (as well as other objects crafted from precious metals. Now I want to address how heavy gems are, and more to the point how heavy objects made from gemstone materials are. For instance, characters may discover a 2′ tall statue made of pure emerald, or a malachite throne. I will then need some method of coming up with a weight so they can determine how (and if) they can transport such a treasure. To do this, I need to come up with a density for the various gems.

The actual density of various gems ranges roughly from 1.0 to 5.0 g/cm^3, with 2.5 to 3.5 being more common. For convenience, I want all gems (of the same size) to weigh the same. For that to happen all gems must have the same density. It would be easy enough to pick a medium value and calculate weights given the mass of each size category gem. But instead, I am going to fudge the numbers a bit.

I want an average gem to be 1″ in diameter and weigh 1/20 of a pound. If I select a medium density value, I would have to adjust either the diameter (size) or the weight to compensate. Instead, I chose to use the size and weight to calculate the density and apply that density to the rest. Given 1″ diameter and 0.05 pounds, I get a specific density of 2.6106476 g/cm^3 (assuming exactly 28 grams per ounce). I would have chosen an average density of 3.0 so this isn’t fudging things too badly.

I originally wanted gems to weigh twice this much (10 average size gems to a pound) but that would have required a specific density of 5.2212952 g/cm^3, which I considered to be higher than I thought was acceptable. I may make this change anyway, as I prefer ten [average] gems to a pound over 20 to a pound. The numbers just seem less bulky.