Classic blue jeans traditionally used copper for their reinforcing rivets.
Copper rivets were placed at points that received heavy strain, including pocket corners and other seams. Jacob Davis developed the riveted trouser idea and worked with Levi Strauss to patent it in 1873. The rivets helped make work trousers more durable for miners, laborers, and other workers.
The rivets were especially useful because early jeans were designed as hard-wearing work clothing rather than fashion garments. Denim supplied a strong woven fabric, while the metal fasteners reinforced locations where stitching alone could fail.
Modern jeans may use different construction methods, decorative rivets, or imitation rivets. That variation can cause confusion, but copper is the traditional material associated with the original reinforced blue-jeans construction.