A very sensible sharpening system..

[D_W]

Unglazed portion of a ceramic cup (turned upside down) also works.

Don't tell the people at my office to turn the cups over to look....they'll probably think they just have some strange dirt on them. Anything that looks like glass or hardened steel with a polish is fine as a polished steel, and unglazed ceramic is pretty much a less precise spyderco medium (they do seem to vary a lot).