I totally agree with @misken advise.
Most materials don’t have much of a range from when it starts to mark, to where it goes to black. So a grayscale isn’t a realistic target on slate or metal.
Most materials you have to look at like it’s a printing press, it either marks or it doesn’t. That’s the most dependable way, that involves using a dither of some type.
You can check out some of Nicky Norton for an idea of what is possible. He has a number of lasers. Of course it depends on the material.
These are done on a 60W MOPA fiber laser using aluminum or stainless coated cards.
And on leather with a 5W diode.
Compares Stucki and Jarvis.
These are only to show you that you can do some incredible reproductions using a dither.
Metal is much the same way. Most of these materials either damage the material or cause an oxide layer. A few materials can do this, many can’t.
Different kinds of slate lase to black and white.
It’s also natural stone with all kinds of debris can end up. So it may not engrave flat everywhere. I did a 3d type image and had mountains in it from something in the slate.
If it’s too hot it will melt, as will most things. Sometimes these get hot and just crack. I have a pile of rocks that I lase out of curiosity and find they are mostly different.
Good luck.
