I found these stones at a local yard supply place, and they engrave nicely. The contrast is not as high as slate, but it’s still pretty good.
The summary is try speed 900, power 20, 270 LPI. This produces a nice image and adds minimal texture to the rock. Above 270 LPI I did not see any obvious resolution benefit, but I did not test extensively.
Below that power, contrast drops off rapidly. Above, you get hardly any extra contrast, but the extra power makes the engraved area much rougher to the touch. The coloration also becomes irregular with higher power. I guess you dig in enough that you start to deal with irregularity in the rock itself, as opposed to just damaging the polished layer.