This tutorial explains how to create a basic nail which could be added to any rusty/dirty interface. The main purpose is to make an interface look more detailed.
Create a new layer. Using the Elliptical Marquee tool make a round selection the size of a nail (6-10 pixels). Set foreground color to R: 140; G: 140; B: 140. Fill the selection using the Paint Bucket tool.
Create a new channel. Using the Pencil tool draw two white pixels near each other as shown in the picture.
Using the Rectangular Marquee tool make a selection around those pixels as shown in the picture. In the main menu, select Edit > Define Pattern.
Remove the selection. From the main menu select Edit > Fill and the Fill dialog will popup. Set Content to Pattern and click OK.
Load selection from the current channel. Create a new layer and fill selection with black using the Paint Bucket tool. Group the new layer with the previous layer. Set Blending Mode to Soft Light.
Duplicate layer created in the previous step. Invert it (Ctrl+I) and shift it one pixel to the left. Link both layers, with black and white "checkerboards".
Having one of the "checkerboard" layers active press Ctrl+T to rotate it. By rotating one layer, the second linked layer will be rotated as well. Rotate them to a random angle and press Enter to confirm rotation.
Done!