"Good swords were very expensive. They did not rust, but went dark and stayed sharp."

Don't know who said that, but they rusted if they weren't cared for. You can see pictures in lots of history & archaeology books of swords with blades rusted partly or entirely away. And no blade will stay sharp if you use it, or don't care for it properly. But a sword won't rust if sunk in a well or a river and left; it's the alternate wetting and drying that rusts iron.

I've got a knife in the kitchen now with the blade tarnished brown. Depends on the chemistry of the steel & of whatever tarnished it. It's a slick blade -- polished before it was brown -- and has a brown shine that might look like varnish.

"Weapon knife" does sound awkward. Weapon gun? Weapon axe? The words don't swing much. However I've heard of war swords, and fighting knives, though that's a modern term I believe, and a little odd; sounds like "pugnacious blades." I've got my grandfather's sticking knife, and some carving and whittling knives. Hunting knife, skinning knife, paring knife, mostly there's an "-ing" but there's butcher knife, and a neck knife is a small knife you carry on a cord round your neck. Interesting how words hook together. Is this what linguistics deals with?

clint