These are four NPC crystals that are part of the Gateway to the Deep quest. They are located on a

Shemza

Lurza

Lurza tells you, "You must bring to me a shard of the Radiant Crystal. Only when Shemza sings your name to the lord of the deep will you gain access to that Crystal."

Dmesne

Dmesne tells you, "I require a shard of the Dazzling Crystal. Until you have acquired one of these from my brother Lurza, I will have naught to do with you, little one."

Rhujun

Rhujun tells you, "A shard of the Brilliant Crystal is all I require. Until I have it, you are not fit."