@ the OP: Don't stain the rock castings. First paint the entire section of rock with one solid color base--this would be something like a house color you might buy at your local building center (Home Depot, Lowes, Menards, etc), and then work on your detailing over that. Reason: Staining individual castings joined between by fillers of plaster or whatever you use end up looking very mottled and inconsistent.
That said, if you DO try the stain alone method first and it doesn't come out well, you can always just paint over the whole thing and start over again.
As to capturing the flavor of the groups of rocks. India ink (black) is thin, and can be matted back up. But make sure your do that before the drops of ink show up as drips--the pattern of drips--which can't be mopped up once they dry.
However, if they do, and can't be mopped up gracefully, just paint over the whole (smaller) section and try again.
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The other: Dry brushing is your friend. Just a hint of your lightest color (white or near white, in most cases) will pick out the edges of the rocks...and the eye picks them out too. They will be the brightest of the bright daylight effects, and they might carry on across most of your rocks which would face the sun. You won't need or want them on the bottom sides, where rock would be shadowed. So apply in the right spots, but not in the wrong spots.
