Hi Hans
I would recommend calling Ikonics Imaging (Photobrasives) for some samples of their film. I always seem to disagree with Dave lately but I don't think a plotter is the way to go in the business you are trying to get into because I would imagine that the rocks are small and not large boulders. Plotters are just no good with small detail. Although film seems expensive it is really not when all factors are involved. In 100 foot rolls the cost of film is approximately $0.06 per square inch for 5mil self adhesive film. If you do enough volume this can come down to less than $.05 a square inch. On a rock that is going to be something a tourist could carry with them it would probably have an area of less than 4x4 for engraving. That means the film just cost me less than $1.00 for 16 square inches. In that area I can but graphics text small detail anything I want without having to worry about weeding and all of the other things involved it plotters. Time involved for artwork is the same for both ways, washout and developing time for the film is less than 1.5 minutes. That is the same time that it would take just to print on the plotter.
By the way I have a plotter and other than for very large jobs I cannot figure why people doing small work find them better. My guess is that when you first start the cost of film is expensive compared to plotter vinyl and thoughts of large volumes are not considered. Again this is just my $0.02! I have done pebbles through boulders but most of my work is crystal, glass and metal.