I came up a technique for reproducing the look of Damascus Steel, useful for up-market swords. (Valyrian steel in the Game of Thrones world is supposed to have a similar look).
1. Draw a plain blade, making the outline with the lasso tool and filling it with grey. Suggest the fuller (groove) down the centre, and make one side slightly lighter, to suggest the profile of the blade, with one edge catching the light more than the other.
2. Create a new layer above the blade and draw some squiggly lines across it in two shades of grey, one lighter and one darker than the original blade below.
3. Go to 'Filter' and then 'Liquify', and then smudge and push the light and dark grey lines around each other until swirls and patterns are generated.
4. Crop off the edges so that the pattern is only within the blade, and reduce the opacity of the layer to about 30%, or whatever looks best.
(I'm told that a clipping mask could be used here, alternatively)
5. Create a new layer with the mode set to 'vivid light' and add highlights with the colour set to a very pale blue.