You can look here to learn which stats the stones we will use in the game increase.
Equipment Types
Gloves
Pants
Sleeves
Cape
Gem Stats
Gem Stat Effect
Ruby STR 
Damage / Hitrate
BlackMoon DEX Evade / Hitrate
Beryl CON Defense / Magic Resistance
Diamond WIS Magic Power
Sapphire INT Magic PowerLarge Gem Status
Gem Stat Value
Ruby STR 
BlackMoon DEX Beryl CON Diamond WIS Sapphire INT EQUIPMENTS
Type Slot
Weapons First of Slot
Armor First of Slot
Shield First of Slot
Guard Arm First of Slot
Gem Stats for Equipments
Gem Stat Effect
Ruby STR Damage / Hitrate BlackMoon DEX Evade / Hitrate Beryl CON Defense / Magic Resistance Diamond WIS Magic Power Sapphire INT Magic PowerLarge Gem Status
Gem Stat Value
Ruby STR BlackMoon DEX Beryl CON Diamond WIS Sapphire INT