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Here is some information regarding D&D gems. I started with the official D&D information and expanded on it. I use this information in my games.
Standard gems carried by adventurers are 1/4″ in diameter. They are carried in small leather or cloth bags tied securely with a string or leather strap. Unless your character is carrying an extremely large number of gems, their weight is insignificant and you do not have to keep track of it. The information listed here is provided for those rare occasions when your character may need to carry an exceptionally large gem or a very large number of gems.
In the real world, gem sizes do not determine weight. The density of gems varies from very light stones like opal to especially heavy stones like sphalerite or zircon. To keep it simple for D&D I am keeping the weight of all gems the same and only vary the value depending on gemstone type and size. The price per carat does not increase smoothly with increasing size. (One 4 carat gem is more valuable than four 1 carat gems.) To allow for larger stones in my game they don’t increase quite as fast as they do in the real world. I justify this by saying that larger stones are not as rare in my world.
Note regarding uncut gems.
When a piece is uncut it is called “rough”. The process of cutting and polishing it results in size and weight loss. In order to overcome the weight loss in addition to the cost of cutting, a cut gemstone is more valuable than rough. The yield from even well-shaped rough material is typically only about 25%.
To make it easy, figure a rough gem is the same value as shown below but it is at least 4 times as heavy and its size is at least 1.25 larger than indicated. It is also irregular in shape and not as “pretty”.
All gems listed here are round faceted and have the same depth as their diameter making them roughly spherical (think 20 sided dice).
|Size||Weight||Value||Number of gems this size that will fit in one cubic foot of space|
|6″ Dia. (size of small human skull)||31,000 carats||14 lbs.||Base value x 50,000||8|
|3″ Dia.(large fist size)||4,856 carats||2 lbs.||Base value x 5,000||64|
|2″ Dia. (que ball size)||1,133 carats||1/2 lb.||Base value x 1,000||216|
|1″ Dia. (Very Large – fits snugly into an empty eye-socket)||145 carats||0.064 lbs. (16 = 1 lb.)||Base value x 100||1,728|
|1/2″ Dia. (Large – marble size)||18 carats||0.008 lbs (125 = 1 lb.)||Base value x 10||13,824|
|1/4″ Dia. (Medium – standard size)||2 carats||0.001 lbs.(1,000 = 1 lb.)||base value||110,592|
|1/8″ Dia. (Small)||0.3 carats||0.000125 lbs. (8,000 = 1 lb.)||base value / 10||884,736|
All gems weigh 110 lbs. per cubic foot (should you be so fortunate as to need to know this).
|agate (banded, eye, or moss )||10 gp|
|freshwater (irregular) pearl|
|rock crystal (clear quartz)|
|quartz (rose, smoky, or star rose)|
|garnet (red or brown-green )|
|pearl (white, golden, pink, or silver )|
|spinel (red, red-brown or deep green)|
|garnet (violet )|
|spinel (deep blue)|
|topaz (golden yellow )|
|opal (white, black, or fire )|
|corundum (fiery yellow or rich purple)|
|sapphire (blue or black star )|
|emerald (clearest bright green)||5,000 gp|
|diamond (blue-white, canary, pink, brown, or blue)|
Here is very nice Gemstone Generator that gives you a random selection of gems based on the starting GP value selected. This is good for those times that the characters find a bag of gems: