Haha I need to get some abalone shells!! But yes, we made it! When I was in high school we made reef balls which are placed in the ocean off the coast of Jacksonville as an artificial reef. We used 3 different size forms, 3 foot, 4 foot, and 5 foot tall and wide. Diver bouy balloons inside to make the holes and then the cement trucks came and we used mallets to get the air bubbles out. Just like this bowl. I actually found a website that had found a pretty good process with the bowls and securing the molds with bolts to ensure uniform thickness.
For color, various shades of a darker grey. Not too dark but to contrast the silver travertine. I found a etching stain at Lowes and it is a grey blue color but I think it is too light.
Oh and I forgot. It is 21" across at the top. We found one place that sells the plain concrete bowls locally but they are $125 each and the smallest is 24" and that will be too big for our columns.