What specific tools are needed to mark metal with a rotary engraver?

It is necessary to know what kind of metal you are engraving. The harder the metal, the different kind of diamond you may need; maybe a burnishing diamond instead of a diamond-drag for hardened metals. 

For instance, some manufacturers offer a diamond burnisher specifically for stainless steel. This has become a popular tool with our customers because it marks the stainless well and it has a longer life than a normal diamond-drag does on the stainless. Softer metals can be engraved by normal diamond-drag engraving tips. Making sure that the diamond will mark the metal is the biggest challenge, but practice makes perfect. Practicing with different diamond tips on different metals before engraving a final product helps with this process.

   —U-Marq