The MicroSlice will engrave onto darker wood and plastics. To engrave lighter colours you will need to use a process called Black-masking. Black-Masking involves laying a sheet of black paper over the surface to be engraved. The engraving process is run once, the black paper is removed, and then the process is ran again.
The last video on the project page demonstrates Black-masking.
The MicroSlice will leave no marks on metal or glass.