The base value of gems found in a treasure can be determined in whole or by lots of 5 or 10 stones by rolling d%:
Die Score Base Value Description (or Size) <Enc.(theory)>
01-25 10 g.p. each Ornamental Stones very small 1
26-50 50 g.p. each Semi-precious Stones small 1
51-70 100 g.p. each Fancy Stones average 2
71-90 500 g.p. each Fancy Stones (Precious) large 3
91-99 1,000 g.p. each Gem Stones very large 4
00 5,000 g.p. each Gem Stones (Jewels) huge 5

Value of a gem depends upon its type, quality and weight.
A huge semiprecious stone - carnelian, for example - is worth as much as an average gem stone, quality being equal.
Size may vary from stone to stone, a 50 g.p. ornamental stone being of above average size,
while a 50 g.p. gem stone would most likely be very small.

Ulaa (goddess of gems)

Encumbrance Value of Gems
very small & small = 0.1#
average = 0.2# (2 is an easier number to work with than 3)
large = 0.3#
very large = 0.4#
huge = 0.5 #

As a quick rule of thumb, all gems start out at average size: increase this or decrease this by one size category for each increase or decrease in base value.

<go with the Enc.(theory) above: this is a good example of where game school takes precedence over realism/simulation.>
<it ain't like that in real life - thanks to bloatedblowfish for pointing this out!>

Increase Or Decrease Of Worth Beyond Base Value:
If you do not place specific value on each gem in a treasure, showing rather the base value of each gem instead, then variation in the worth of each stone should be allowed.
This variation will generally result in some increase, although there is a chance for decreasing value as well (see below).
To find if a gem increases in value, roll a d10 for each stone, and consult the table below:
Die Result
1 Stone increases to next higher base value; roll again ignoring results above 8. 
Stones above 5,000 gold piece value progress as follows: 
10,000 GP, 25,000 GP, 50,000 GP, 100,000 GP, 250,000 GP, 500,000 GP, and 1,000,000 GP -- the absolute maximum. 
No stone may increase beyond 7 places from its initial base value.
2 Stone is double base value. Do not roll again.
3 Stone is 10% to 60% above base value. Roll d6 to find new value. Do not roll again on this table.
4-8 Base value shown is unchanged.
9 Stone is 10% to 40% below base value. Roll d4 to find new value. Do not roll again on this table.
0 Stone decreases to next lower base value; roll again on this table, ignoring any result below 2. 
Stones below 10 gold piece value are: 5 GP, 1 GP, 10 SP, 5 SP, and 1 SP. 
No stone may decrease beyond 5 places from its initial base value.

When base value only is known, use the table above, and roll for each stone.
Stones for which a 1 or a 0 is rolled must be diced for again on the table, but all others are excluded from such rolls.
If large numbers of stones are in question, it is suggested that they be diced for in groups in order to make the process less time-consuming.

transparent (no notation)
translucent (italics)
opaque (*)

ORNAMENTAL STONES, Base Value 10 g.p.:
d12 Name Reputed Magical Property Weight
1. Azurite*: mottled deep blue
- -
2. Banded Agate: striped brown and blue and white and reddish <compress image>
Restful and safe sleep -
3. Blue Quartz: pale blue - -
4. Eye Agate: circles of gray, white, brown, blue and/or green
Restful and safe sleep -
5. Hematite*: gray-black
Aids fighters, heals wounds -
6. Lapis Lazuli*: light and dark blue with yellow flecks
Raises morale, courage -
7. Malachite*: striated light and dark green
Protection from falling, Wards off spells, evil spirits, and poisons -
8. Moss Agate: pink or yellow-white with grayish or greenish "moss markings"
Restful and safe sleep -
9. Obsidian*: black - -
10. Rhodochrosite*: light pink - -
11. Tiger Eye: rich brown with golden center under-hue
- -
12. Turquoise*: light blue-green
Aids horses in all ways (but stone shatters when it operates) -

SEMI-PRECIOUS STONES, Base Value 50 g.p.:
(d20, reroll)
Name Reputed Magical Property Weight
1. Bloodstone*: dark gray with red flecks () Weather control -
2. Carnelian*: orange to reddish brown (also called Sard)
Protection from evil (Benefits wisdom) 5
3. Chalcedony*: white <image> Wards off undead -
4. Chrysoprase: apple green to emerald green
Invisibility -
5. Citrine: pale yellow brown
- -
6. Jasper*: blue, black to brown () Protection from venom -
7. Moonstone: white with pale blue glow () Causes lycanthropy -
8. Onyx': bands of black and white or pure black or white () Causes discord amongst enemies -
9. Rock Crystal: clear
- -
10. Sardonyx*: bands of sard (red) and onyx (white) or sard*
Benefits wisdom + Causes discord amongst enemies -
11. Smoky Quartz: gray, yellow, or blue (Cairngorm), all light - -
12. Star Rose Quartz: translucent rosy stone with white "star" center - -
13. Zircon: clear pale blue-green ( - -

FANCY STONES, Base Value 100 to 500 g.p.:
(d20, reroll)
Name Reputed Magical Property Weight
1. Amber: watery gold to rich gold (100) Wards off diseases -
2. Alexandrite: dark green (100) Good omens -
3. Amethyst: deep purple (100)
Prevents drunkenness or drugging -
4. Aquamarine: pale blue green (500) () Wards off foes -
5. Chrysoberyl: yellow green to green (100) () Protection from possession -
6. Coral*: crimson (100) Calms weather, safety in river crossing, cures madness, 
stanches bleeding
7. Garnet: red, brown-green, or violet (the most prized) (100) (500) () - -
8. Jade: light green, deep green, green and white, white (100)
Skill at music and musical instruments -
9. Jet*: deep black (100)
- -
10. Pearl*: lustrous white, yellowish, pinkish, etc. to pure black (the most prized) (100) () (500)
- -
11. Peridot: rich olive green (Chrysolite) (500) <>
Wards off enchantments (Wards off spells) -
12. Spinel: red, red-brown, deep green, or very deep blue (the most prized) (100) (500)  - -
13. Topaz: golden yellow (500)
Wards off evil spells -
14. Tourmaline: green pale, blue pale, brown pale, or reddish pale (100)
- -

<peridot, chrysolite: is that 2 different gems? if so, make 2 different entries>

Quick Generation (100 gp gems)
1. Amber
2. Alexandrite
3. Amethyst
4. Chrysoberyl
5. Coral
6. Garnet
7. Jade
8. Jet
9. Pearl
10. Spinel
11. Tourmaline

Quick Generation (500 gp gems)
1. Aquamarine
2. Garnet (violet)
3. Pearl (pure black)
4. Peridot (Chrysolite) <?>
5. Spinel (very deep blue)
6. Topaz

GEM STONES, 1,000 or more g.p. Base Value
(d20, reroll)
Name Reputed Magical Property Weight
1. Black Opal: dark green with black mottling and golden flecks () - -
2. Black Sapphire: lustrous black with glowing highlights (5,000) Aids understanding of problems, kills spiders, boosts magical abilities -
3. Diamond: clear blue-white with lesser stones clear white or pale tints (5,000) () Invulnerability vs. undead -
4. Emerald: deep bright green () Wards off foes -
5. Fire Opal: fiery red
- -
6. Jacinth: fiery orange (Corundum) (5,000) Luck travelling, wards off plague, protection from fire -
7. Opal: pale blue with green and golden mottling
- -
8. Oriental Amethyst: rich purple (Corundum) Prevents drunkenness or drugging -
9. Oriental Emerald: clear bright green (Corundum) (5,000) - -
10. Oriental Topaz: fiery yellow (Corundum) Wards off evil spells -
11. Ruby: clear red to deep crimson (Corundum) (5,000)
Gives good luck -
12. Sapphire: clear to medium blue (Corundum) () Aids understanding of problems, kills spiders, boosts magical abilities -
13. Star Ruby: translucent ruby with white "star" center
Gives good luck -
14. Star Sapphire: translucent sapphire with white "star" center () Protection from magic,
Aids understanding of problems, kills spiders, boosts magical abilities

Note that corundum gems are extremely hard.
They should get a save bonus (+2? +4?) vs. crushing blow & normal blow -- see DMG.80.III & DMG.81.DMS.

Quick Generation (1000 gp gems)
1. Black Opal
2. Emerald
3. Fire Opal
4. Opal
5. Oriental Amethyst
6. Oriental Topaz
7. Sapphire
8. Star Ruby
9. Star Sapphire
0. Roll again

Quick Generation (5000 gp gems)
1. Black Sapphire
2. Diamond
3. Jacinth
4. Oriental Emerald
5. Ruby
6. Roll again

<with regards to quick generation, an extended table could be done, that has individual entries for each of the color variants.
the color of a gem is important when one wants to TALK with azer>

Dang, Treebor!

That's impressive to me, especially because I love gemstones, mainly the colored ones...perhaps set off by diamonds or pearls... 
I'd love a chest full of diamonds, corundum and other colored gemstones to admire even if those stones were a penny a carat!