High ultraviolet oxidizes woods and bleaches the color out of some gemstone. Heat bakes the protective oils out of most hardwoods and weakens adhesive bonds. Hand wash blades when necessary with non-abrasive gentle detergent, polish blades and fittings with good quality polish like Simichrome. Clean handles and sheaths with damp cloth and buff with soft dry cloth. Protect carbon steel knives with a light coating of hand-buffed wax, not oil.
Oil attracts dust as well as weakens the sheath.
Some carbon steel knife blades are blued. Nitrate bluing is a very thin patina that will wear away, leaving a gray metal finish. They are rust inhibitors, not rust preventatives. Etching is used in the maker's mark and for some cosmetic enhancement. Wood handles usually benefit from a light coating of furniture wax and a good hand rubbing.