Final answer:
The mythological substance created from the tears of the Heliades after Phaethon's death is amber. Amber's historical significance is backed by Thales of Miletus's discovery of its electrostatic properties when rubbed with fur.
Step-by-step explanation:
The substance that was created according to mythology when the Heliades wept following the death of Phaethon is amber. This is not just a mythological element but also a significant substance historically, as the ancient Greek philosopher Thales of Miletus discovered its properties. Thales found that when amber is vigorously rubbed with a piece of fur, it generates a force that attracts the fur. Furthermore, the rubbed amber could also attract other nonmetallic objects, which is an early observation of static electricity.