If you make the sheath out of veg tan leather an alternative is to finish the sheath, including any stains or dyes, then burnish it all over with something hard, rounded and shiny. For example a shiny piece of steel, the neck of a bottle, a bone folder tool even. Burnishing is just rubbing firmly all over the surface with a firm even pressure. The effect on the leather is to compress it slighty, and make it shiny, and stiffer than it was.