Re: [DF 8] On Gemstones and Carats
 

Using DF8 gives you a 1/36 chance of BIGGER!.

So, how rare do you want the gem to be?

Odds of being an N iteration of BIGGER! are 1/(36^N)

To do the math for you:
1: 32
2: 1,024
3: 32,768
4: 1,048,576
5: 33,554,432
6: 1,073,741,824
7: 34,359,738,368

So, to put it in the range of 1 bigger makes it as big as one in 32 gemstones. 2 biggers makes it one in a thousand (or so). A 1-in-a-million gem is in the 4 biggers range. 6 biggers is probably the only gem of its kind in the world. More than that is just ridiculous. This involves no reality-checking. Just crunching numbers from DF8 to see the odds. Also note that there are great overlaps in the size ranges, but as you roll more dice you'll get to a more average result.

P.S. It hurts me to write the word "biggers"

Re: [DF 8] On Gemstones and Carats
 

Nah, you're just looking at the biggest gem in the inner planes, then the biggest gem in the inner and outer planes, and then probably "the heart of the First Dragon/Creator God/Abyss".

On the other side of things, there's "gem stones" as the term is used in jewelery, which is to say "pretty rocks", which can include some that get VERY big.

Quartz: "Well-formed crystals may reach several meters in length and weigh as much as 1,400 pounds (640 kg)."

Jade: "The world's largest pure gemstone quality jade named "Polar Pride" was discovered in British Columbia in the vicinity of Dease Lake. It weighed 18 tones..."

Hematite: "The most spectacular large crystals of hematite--flat plates 6 in. (15 cm) or more across--have been found in metamorphosed Brazilian sediments."

Re: [DF 8] On Gemstones and Carats
 

For stones that large, I suspect that the Carat-based pricing presented in DF8 breaks down. A single Jade boulder weighing 18 tons (90 million carats) would be valued at $162 Quadrillion, which is over 2500 times the GDP of the entire planet. Even a 1400 pound quartz comes in at over $150 Trillion. At some point the pricing scheme must drop the mass^2 component, and plateau at a fixed price per kilo.

Re: [DF 8] On Gemstones and Carats
 

There is a cave in Mexico with impressive gypsum crystals.

Re: [DF 8] On Gemstones and Carats
 

It currently doesn't, despite having a whole bunch of decorative stones and not-really-stone materials that appear in single pieces significantly larger than traditional "gemstones".

Which is probably a decent enough abstraction for DF - it's not like the value per carat isn't completely made up as it is. But quartz, jade, jet, coral, etc and any of the bulk iron or copper ores should (in a more accuracy-concerned game) just be a fixed per pound measurement (SJG measures by the lb, as much as I'd prefer kg).

My point was more "Or the giant stone in the idols eye could be Chalcondy" - still seriously worth it as a giant lump of reasonably expensive decorative material, but not quite so economy imploding as a giant ruby.

Re: [DF 8] On Gemstones and Carats
 

The based on square of carats is largely a function of the rarity of large unflawed stone. For stones where flaws are invisible or irrelevant, it doesn't make sense.

Re: [DF 8] On Gemstones and Carats
 

In real life Earth gemstones in the Middle Ages were polished not cut. And diamonds were not popular due to their lack of color. It was not until the modern cutting methods that created a play of colors from the interplay of light between the facets that diamonds became popular; that and a great publicity campaign by DeBeers.
In the Middle Ages the pricing would have been different than the modern method. It was: What does the customer look like they will spend? Even today there is a strong element of "An item of jewelry is worth what you think it is worth." in all transactions. Back then jewelry and gems were a status indicator and the bigger the rock, the higher your status was presumed to be.
Also, bear in mind that the designation of gems was a lot looser than present day. Green gems were lumped as emeralds but today a distinction would be made between emeralds and other green gems.
Fashon and style made a huge difference in price so you can get away with charging whatever you want when the players buy and giving whatever you want to give when they sell. They do not have to agree with what is offered but there would be, historically, no fixed method of determining a value independant of that.

Re: [DF 8] On Gemstones and Carats
 

So give them a 710 carat zircon. :)

Re: [DF 8] On Gemstones and Carats
 

Several years ago, there was an exhibition in Memphis, TN of artifacts and possessions of the Ottoman Sultans, including the legendary Topkapi Dagger. There were some other gemstones and jewelry that on display that belonged to the earlier sultans, and you could see the difference in gem technology vividly on display. The older items were polished gems, not cut or faceted, they looked like what they were, pretty colored rocks, rather than crystalline translucent sparklers. The newer items, made more recently with more advanced techniques, looked more like what we think of as 'gems and jewels'.

Re: [DF 8] On Gemstones and Carats
 

And going the other way, rubies and sapphires were split up, even though they are the same kind of rock with different impurities. Same with beryl, aquamarine, and emerald.

And of course, several gem "stones" are not really stones in the truest sense. Pearl, jet, amber, opal, etc.