Rathalayan firestones were akin to geodes in size and shape. What made them unique were their inner crystal formations. They were quite similar in makeup to the crystals that Jedi used in lightsabers; however, they were not atomically strong enough nor stable enough to be used in such ways.
When two or more firestones were placed in close proximity to each other, the crystals would begin to vibrate. This vibration created heat, a feature that gave the stones their name.
The firestones were found on Rathalay.