For this post, I drew heavily from the "reputed magical properties of gems" listed in the 1E DMs Guide and turned those suggestions into mechanics. Use these tables to turn any weapon, armor, or mundane item into a treasure (perhaps magical) based on the gem or gems that are encrusted upon it.
Roll 1d6 to determine how many gems are on the item (or multiple d6's if you really wanna go nuts.) There is a 15% chance, for each gem, that it possesses the magical property listed next to it. Multiple gems do stack.
Category of Gem....Roll 1d4
1. Ornamental Stones
2. Semi-Precious Stones
3. Fancy Stones
4. Gem Stones
Ornamental Stones (value is 1d4 x 10 GP).....Roll 1d6
1. Hematite (+1 to hit bonus)
2. Lapis Lazuli (+1 or 5% to retainer morale checks)
3. Obsidian (+1 or +5% on stealth checks)
4. Blue Quartz (light sources last twice as long)
5. Tiger Eye (+1 to initiative rolls)
6. Turquoise (25% more distanced traveled in a day)
Semi-Precious Stones (value is 1d4 x 50 GP).....Roll 1d6
1. Bloodstone (+1 to AC)
2. Jasper (+1 save vs. poison)
3. Onyx (-1 or -5% to enemy morale checks)
4. Zircon (+1 save vs. energy drain)
5. Carnelian (protection from evil lasts twice as long)
6. Moonstone (+1 to saves vs. lycanthropy)
Fancy Stones (value is 1d4 x 250 GP).....Roll 1d6
1. Jade (+1 to reaction rolls)
2. Peridot (+1 save vs. magic)
3. Amethyst (+1 on save vs. intoxication)
4. Amber (+1 save vs. disease)
5. Topaz (+1 save vs. evil)
6. Garnet (take 1/2 damage from fire)
Gem Stones (value is 1d4 x 1,000 GP).....Roll 1d6
1. Diamond (+2 on turn udead rolls)
2. Ruby (+2 on all saves)
3. Emerald (+2 hit points healed per die with curing spells)
4. Sapphire (targets of your spells save at -2)
5. Opal (+2 save vs. petrification/paralyze/polymorph)
6. Jacinth (+2 to damage rolls)
Just for the hell of it, let's say there is a 10% chance that the gem has even greater magical properties.....Roll 1d6
1. +1d4 to AC
2. +1d4 to-hit
3. +1d4 to all saves
4. +3d4 hit points
5. Can cast 1 randomly determined magic-user spell 1d4 times per day (spell level 1-4.)
6. Can cast 1 randomly determined cleric spell 1d4 times per day (spell level 1-4.)
Let's say there's a 5% chance that a gem is
cursed causing the owner to suffer ability loss while the item's in their possession....Roll 1d6
1. -1d4 strength
2. -1d4 constitution
3. -1d4 intelligence
4. -1d4 charisma
5. -1d4 dexterity
6. -1d4 wisdom
