List of Gems and what they do:

Turquoise: +10 Rock, +10 Mimic, +10 Ice 
Emerald: +10 Dino, +10 Plant, +10 Magic 
Diamond: + 5 vs. All Creatures, Extra +5 vs. Steel 
Garnet: +10 Animal, +10 Rock, +10 Fire, +5 Attack 
Pearl: +10 Undead, +10 Thunder, +10 Endurance 
Ruby: +10 Mimic, +10 Fire, +10 Speed 
Sapphire: +10 Flying, +10 Wind, +10 Magic 
Amethyst: +10 Mage, +10 Dino, +10 Ice 
Opal: +10 Mage, +10 Steel, +10 Endurance 
Peridot: +10 Plant, +10 Animal, +10 Holy, +5 Attack 
Aquamarine: +10 Sea, +10 Flying, +10 Ice 
Topaz: +10 Undead, +10 Flying, +10 Speed, +5 Attack