What do you use to pillow the blocks? Angle grinder? Some fancy diamond-bit router?
That stone looks like it'd be fun to work with - around here the water doesn't affect the weight of the stone, but it's likely because most of the limestone here is about 140-160 pounds per cubic foot, where that stone sounds like it's quite a bit less dense (I calculated about 80 pounds per cubic foot).
A lighter stone would make that work much more enjoyable.
Did you do anything special to maintain alignment of the piers? Laser, maybe?
Excellent work.
