Amaretto,
I am not positive I am understanding, are your boulders really tufa (as in peatmoss, perlite mix) possibly making the boulder kind of a scratchy surface? Or is it a little more compact?
Anyhow as to a 'varnish', there are numerous concrete stains that may mimic a wet river slate, or take a look at some of the Behr deck stains, Mason stains, and other wood stains. The Behr especially has a couple of color options that although are made for wood, when applied to concrete, in my experience, will pick of a few different color variences and at the same time it has a egg shell to semi gloss sealant look, definitely able to mimic wet river rock with some protectant features.
Would love to see pictures of what you are doing.
Rock building is definitely rewarding!
Dena

Thanks, Dena

My tufa boulders use an approximation of the posted sandstone mix, about 3:2:3 of Peat, Cement and Sand respectively. I added a hint of yellow/rust concrete dye which. Without the dye, I'd think that when wet, the tufa would look greyish, however with the dye, I'm thinking it looks greyish with a hint of brown. This may be because of the exposed peat moss, but I do have older tufa rocks that are having the same problem.

The funny thing is I now have two boulders stacked on top of eachother, and when it rains you can clearly see where the top boulder is protecting the bottom one from the rain.

I'd like to prevent this as I intend to use them for a waterfall, and this wont render the realistic results I'm hoping for.

When looking at other boulders in the area (I don't know if they're fabricated), but I noticed some had a transparent layer over them which made them look like river rocks. This is the effect I'd like to accomplish. I'll check out Behr and see what I can find.