Those dark bits at the edges? Grime and filth and disgusting-ness. We started sanding them back just before the September earthquake, then we started renovating the bathroom, and decided we should leave the stairs until there would no longer be workmen traipsing up and down them...little did we know that this would take a good two years!!
Anyway, we finished the job last weekend - a bit more sanding, then a few coats of Osmo Polyx-oil (a nice and natural oil - serves the same purpose as polyurethane without the fumes and stickiness). And voila!
We didn't sand them back too much, for fear that they would start to look a bit too new - it's kind of important for us to keep the history in them - the knocks and scraps and dents.