Piedra, Spanish for “rock,” is a close cousin of the English, petrify: “to be very, very scared”. We see the connection clearly if we map the p‑d-r of piedra to the p‑t-r of petrify.
What is the connection between them? Well, when you get scared, you often just freeze: you turn to stone! So next time someone is so scared that they stop in the middle of their tracks, just think, they are just, petrified!
It’s interesting to note that, these words have a whiff, just a whiff, of Sodom and Gomorrah. Remember the classic scene from Genesis: Lot and his wife are fleeing the city of Sin as they are being destroyed, commanded by God to not look back as they run away. But Lot’s wife is so scared that, she turns back onto it and is thus… turned into a pillar of salt. Her fear turns her into a stone (well, salt, but the same concept!). Literally!