雷の召喚石 [ikazuchi no shoukanseki] or 'lightning summon stone' in Japanese.
Pictlogica

Lightning Summon Stone
Buy: - (sell: ? gil)
Obtain: event quests Treasure Excavation and Rescue the Princess
Use: required for EX Alchemy to create Ramuh +1
Type: Alchemy Material, Rarity: ★★★
Description: Material for EX Fusion and EX Alchemy synthesization

Lightning Summon Stone+1
Buy: - (sell: ? gil)
Obtain: event quests Treasure Excavation and Rescue the Princess
Use: required for EX Alchemy to create Ramuh +2
Type: Alchemy Material, Rarity: ★★★★
Description: Material for EX Fusion and EX Alchemy synthesization

Lightning Summon Stone+2
Buy: - (sell: ? gil)
Obtain: event quests Treasure Excavation and Rescue the Princess
Use: required for EX Alchemy to create Ramuh +3
Type: Alchemy Material, Rarity: ★★★★★
Description: Material for EX Fusion and EX Alchemy synthesization
Category: Item