Engraving materials besides wood

Engraving into metal with a relatively low-powered laser (diode) is not possible, but might have some success with marking using Cermark or similar product. Cermark is kind of the industry standard, though it’s pricey. Dry Moly lube works well (though not quite as dark) and is significantly cheaper.

There are a few other products and techniques you will find that could work for your setup if googled.