Belt   + Amethyst = +Mana based on clvl
Belt   + Diamond  = +Attack rating based on clvl
Belt   + Emerald  = +Defense based on clvl
Belt   + Ruby     = +Life based on clvl
Belt   + Sapphire = Slows Target
Belt   + Topaz    = +Maximum Damage based on clvl
Belt   + Skull    = +All Skills

Gloves + Amethyst = Increased Chance of Blocking
Gloves + Diamond  = Increased Attack Speed
Gloves + Emerald  = Chance of Open Wounds
Gloves + Ruby     = Life Stolen per hit
Gloves + Sapphire = Chance of Deadly Strike
Gloves + Topaz    = Mana Stolen per hit
Gloves + Skull    = Chance of Crushing Blow

Boots  + Amethyst = Teleport Charges
Boots  + Diamond  = Faster Run/Walk
Boots  + Emerald  = Decrepify Charges
Boots  + Ruby     = Blaze Charges
Boots  + Sapphire = Slower Stamina Drain
Boots  + Topaz    = Slow Missiles Charges
Boots  + Skull    = Shock Wave Charges

Helm   + Amethyst = Valkyrie Charges
Helm   + Diamond  = Summon Skeletal Mage & Skeleton Mastery Charges
Helm   + Emerald  = Death Sentry Charges
Helm   + Ruby     = Fire Golem & Golem Mastery Charges
Helm   + Sapphire = Summon Dire Wolf Charges
Helm   + Topaz    = Iron Golem & Golem Mastery Charges
Helm   + Skull    = Revive & Skeleton Mastery Charges

Weapon + Amethyst = Twister Charges
Weapon + Diamond  = Molten Boulder Charges
Weapon + Emerald  = Poison Explosion Charges
Weapon + Ruby     = Meteor Charges
Weapon + Sapphire = Glacial Spike Charges
Weapon + Topaz    = Chain Lightning Charges
Weapon + Skull    = Bone Spear Charges

Torso  + Amethyst = Faster Cast Rate
Torso  + Diamond  = Faster Hit Recovery
Torso  + Emerald  = Blade Shield Charges
Torso  + Ruby     = Attacker Takes Damage based on clvl
Torso  + Sapphire = Chilling Armor Charges
Torso  + Topaz    = Attacker Takes Lightning Damage
Torso  + Skull    = Reduce Damage %

Shield + Amethyst = Magic Find
Shield + Diamond  = Gold Find
Shield + Emerald  = NOTHING! (bug?)
Shield + Ruby     = Absorb Fire
Shield + Sapphire = Absorb Cold
Shield + Topaz    = Absorb Lightning
Shield + Skull    = depends on the ring & amulet used:
   magic:   Knockback and Attacker takes damage
   rare:    Same as above, adds Ignore Target Defense
   crafted: Same as above, adds Prevent Monster Heal
   unique:  Same as above, adds Freezes Target