I've got supplements from around half a dozen different systems, each with their own way of describing and assigning value to various gemstones. Pretty much all go into far too much detail, and assign too much value to things like feldspar, moonstone and haematite, and, at some stage, give each common stone strange and magical powers that serve more to confuse and cheapen good jewellery. (IMHO, that is). But a haematite set necklace shouldn't really be worth all that much compared with the shiny gems that are DIFFICULT to mine or acquire.
I got the idea of writing up my own ideas when I came across "A Re-Evaluation of Gems & Jewel[le]ry in D&D" in Issue #8 of Dragon Magazine. Other aspects of this article, a collection of tables to detail gem & jewellery hauls, have been discussed in the blogisphere, but I'm going to assault TYPES of gemstones.
So here is my little, simplified guide to gems in a FRPG, to add a consistent flavour...
The Cardinal Gems are the most expensive, favoured gems, desirable by nobility, elves, religions and other decadent types with more money than sense. Therefore, treasure seekers and tomb robbers should remember these, be able to identify them, and seek and collect them over other, less lucrative gems.
They consist, unwaveringly, of the following gems;
Diamond - deep volcanic pipes - kimberlites. Associated with garnet & spinel. Also in weathered alluvial placers.
Ruby - red corundum. Marbles and gem gravel deposits
Sapphire - blue corundum. Similar to ruby, usually found together, with one dominating.
Emerald - green beryl. Granite pegmatites, mica schists, contact metamorphosed limestones.
I tend to add pearls to this, given the extra number of 'things that can kill you easily' existing under the waves in fantasy worlds.
So really expensive jewellery will have some of these stones, or jewellery will be expensive as it has some of these. Identification of some of these gems can be hard, though. Don't be afraid to throw a nice zircon gem where they might suspect a diamond, and spinel and ruby were considered the same until not too long ago - several rubies in Crown Jewels are actually red spinel. And, as these links show, most BIG gems have histories, as they are visible, rare and easily transportable, sometimes even without the rightful owner knowing.
Lesser stones are more tricky. They are still shiny, however, and although of lesser value, still worth a bit to starving tomb robbers. Here's a little table I've been working on for Shiny Things Held By People You've Killed or Robbed;
Item of Jewellery (d12);
1. Anklet - ankle chain
2. Buckle - belt, weapon belt, etc
5. Amulet - usually a minor magical effect or protective function. Note 'minor'.
6. Earring - not exclusive to ears...
10. Clasp - like a brooch, but specialised for holding cloaks, clothes and other things fastened.
11. Diadem - any sort of head jewellery - tiara, crown, headband, etc
12. Integlio - small semi-precious gems carved into cameos
And the gem type involved;
The first three are all chrysoberyls;
1. Alexandrite - changes colour with changes in light - green/yellow or yellow/red.
2. Chrysoberyl - yellow or yellow green
3. Cymophane - 'cat's eye' effect
4. Aquamarine - turquoise colour
And these three are all beryls, different from chrysoberyls;
5. Golden Beryl - strangely, yellow or gold colour.
6. Helidor - greenish yellow
7. Morganite - pink to rose shades
9. Peridot - small and green, easily mistaken for emeralds
10. Sunstone - bright yellow, red, pink colours, displays aventurescence
11. Zoisite - can be Blue, Green, Brown, Pink, Yellow, Clear (d6)
13. Black Tourmaline (Schorl)
14. Coloured Tourmaline - Pink, Green, Clear, Dark Yellow (d4)
15. Spinel - Red, Blue, Black, Green, Mauve, Clear (d6)
19 & 20. JACKPOT! Give them one of the Cardinal Gems.
I'm slipping behind on this A to Z thing, but will go for some slightly shorter posts to try and catch up, if I can.