Beach pebbles/rocks

[WarDog]

Blueish and smooth is probably sedimentary. Not an ideal choice. Neither is metamorphic, can contain metals. Best bet is rock forged from 'liquid hot magma' (Dr.Evil voice), which is Igneous.

[GoFish]

What are you planning on using these rocks for btw? Been there done that with beach rocks. Are you wanting more biological filtration? Or wanna use them as a substrate in your display?