Serendipitous staining

Discoloration usually devalues a vintage pen or pencil, and dramatically. Which is why I was able to buy this Mandarin Senior pencil for next to nothing -- around $40, lacking the clip and clip screw, as I recall. But look at how handsomely mottled it is! Like real yellow tortoiseshell, once cleaned up and polished. Still no idea what did the staining; it looks as if the pencil lay in some sort of liquid, since the darkening is predominantly on one side. In any event, I'm not going to try to stain a pen to match.