I bought some of the stuff that Lasersketch calls Absolute Black Marble. However, since I had read here that it is most likely not marble, I put it to the acid test. I put pure vinegar on a piece and it had absolutely no effect. I tried the same test on some stone I know to be marble and sure enough, the acid left a nasty blotch on it.
However, I am not complaining though. This material is some kind of granite and it engraves beautifully. I engraved these pieces at 50s/35p at 300dpi processed with Photograv 3.0 using the generic Black Marble parameter. These photos, due to my crappy lighting do not do the engravings justice. The subjects are just of people and a dog I downloaded from the internet. I did preprocess them some in Photoshop.