From The Heroes of Might and Magic III wiki

Artifacts equipped in this slot usually give a bonus to Spell Power, but there some exceptions. The artifacts equipped in this slot are:

• Breastplate of Petrified Wood
• Rib Cage
• Scales of the Greater Basilisk
• Tunic of the Cyclops King
• Breastplate of Brimstone
• Dragon Scale Armor
• Armor of Wonder
• Armor of the Damned
• Power of the Dragon Father
• Titan's Cuirass