The sink is a box made of basswood, covered all around the sides with a strip of Rigid Wrap. I've used the product in several projects. Here's a link to the manufacturer's page, with information about the product.
I wanted to be sure the box stayed firmly glued together, and also thought the Rigid Wrap would be a good base for the faux stonework.
Here's a closeup of the sink. The bits of white you see are from the plaster, I wanted to take the picture before it got dark and I didn't have time to wash the plaster bits off.
When the plaster was to my liking, I stippled it a little with a small stenciling brush, then I patted it a bit with my finger so it wouldn't be too dimpled or lumpy.
When the plaster was dry I began to paint. - Remember, as long as your plaster,(drywall compound, stucco, etc.) is unpainted, you can smooth it or fix any cracks, or add more plaster with no problem.
The colors I used to create the stone look were: Payne's Gray, Medium Hauser Green, a dark brown and a warm white.
I described Payne's Gray in the timbering section. You can use any kind of off white when you need to lighten a stone color, just don't use plain regular white. It just doesn't give the right effect.
Terre Verte, or Green Earth, adds a nice touch to painted stonework. I didn't happen to have any handy, so I used the Medium Hauser Green instead, which worked very well.
I squirted a bit of each color onto my palette, which happens to be a foam plate. I took a dab each from my Payne's Gray, warm white, and half a dab from the green and brown, then I swirled them together till they were somewhat mixed, but splotchy. I could then see what color I needed more of, and I added more of the warm white, then a little of the gray, etc., always keeping the color on my plate not quite mixed. Having the color all smoothly mixed on the plate is bad, so don't do that. if your color has been over mixed, add more of the colors so it'll be splotchy again. If you try it, you'll see what I mean.
Next I started putting paint on the sink, adding dabs of color, then spreading the color around with a stippling movement - never brushing it on. I'dd more paint as needed, sometimes adding more green, brown or off white to my mixed swirl of paint on the plate.
The stone effect you see on the sink is more the result of the paint than the plaster, the plaster just helps a little.
Stone sinks come in different shades, grayer, browner, greener, whiter. Some are smooth, others rougher. My wash house sink is supposed to have a rougher surface than perhaps some stone kitchen sinks.
Don't be afraid to experiment with faux painting techniques. Use scrapwood if you have it, paper or cardstock if you don't, but plain paper will absorb the paint differently and sometimes results will look different than they would on wood.