While @jkwilborn did a good job answering a lot, this one was a bit left out and I have some experience. Stainless steel works fine, it’s just a matter of dialing it in. I don’t have the 20W, only the 10W, but I’ve done things like stainless steel spoons, bottle openers, etc. Aluminum bare is a no-go from what I’ve experienced, however painted or anodized works really well and pretty fast. Concrete I haven’t done, but stone seems to take well, especially slate and brick. Ceramics? SOME glazes will take a laser with no covering, but for the best results, use titanium dioxide. It’s commonly used as a white pigment, and it’s what makes the “Norton white tile method” work with white spraypaint. However, I found just buying the powder cheap online and mixing say 3-4 tablespoons with 8-12 oz of 99% IPA in a cheap spray bottle works great. Just keep shaking it so it doesn’t settle for an even coat.