@Filippo Hi there... a friend of mine have a laser engraving printer...
1) First of all it is too slow for a 20cmx20cm (clock-draw-latin-letters) needed about 9 hours for engraving at wood
2) You can engrave almost at any material... but need several tests... (he tried at leather, wood, silicon plastic...) ofcourse no at glass, mirror or metalic surface....
General I suggest to select one with plastic-pvc-windows (with color of laser) arround it - because is dangerous... and may damage your skin, also it was very slow.... and need several test to be sure for the result ...