The McMaster-Carr catalog has scribes of all kinds, steel, carbide, and diamond, for not much money, but if I actually need to scribe something in steel or anything softer, a sharpened 1/8" tungsten (2% any-ated, not pure) kicks ass for a little bit of nothing, but, as kermdawg said, it takes a pretty deep scribe to be seen clearly with your hood down. In a situation that critical, you might not want to create a sharp gouge in your material.
Unless you're building satellites for JPL, it's probably not that critical. My rule is to use the easiest method that meets my goal. If you need a really tight line, a silver streak shows up while torching or tacking, and it's reflective enough to show up while welding. For a rough line, soapstone is just fine on carbon, but harder to spot on stainless. I've even used artists' colored pencils in a pinch.
Wow. I really rambled on that post.
Steve