Many metal, plastic and wood items can be engraved. Yes, usually on these pieces only an indent is done on the surface of the item. The engraving usually has no contrast with the surface material, but is permanently in the metal, which is what you want. Engraving is the process where the metal is either plowed to each side using a diamond drag engraving tool or a rotating carbide cutter to cut into the surface. No color is used to fill in the letters to give contrast.
Some people expect engraving to be like printing - very visible.
Engraving is more subtle in appearance as light reflects differently off of the engraving as compared to the metal surface. Silver items are the hardest to see the engraving because of the highly reflective nature of silver or chrome metals. Also, the harder the metal the less depth to the engraving, also making less contrast to the engraving. Gold surfaces show the engraving better. Yes, many engraving operations can only engrave on flat surfaces.