A spark from mankind's original fire, the Flame of Prometheus. Its heat forged Excalibur, and that's what we'll need to put it back together.
Before the Curse
Prometheus created mankind's original fire, the Flame of Prometheus. The Flame is magical, powerful and inextinguishable. When Merlin wanted to find a way to cut his powers away from him, he sought out the Flame of Prometheus to turn the Holy Grail into a sword. However, once this happened, the fire turned into a single spark, and wouldn't be reignited again for centuries. ("Nimue")