Here's why:

* Density and Stiffness: Sound travels as vibrations. Denser materials with stronger bonds between atoms allow these vibrations to travel more quickly. Diamond is incredibly dense and its atoms are tightly bound together.

* Elasticity: Diamond is very elastic, meaning it can deform and return to its original shape easily. This ability to spring back quickly helps sound waves propagate rapidly.

While other solids like steel and aluminum also conduct sound well, diamond outpaces them all.