In Episode 10 of Uchouten Kazoku (5:34), I saw Shimogamo Mother taking two stones from a pot, and striking them against each other before Shimogamo Yaichirou went out.
What does it mean? Is it some kind of supernatural belief?
In Japanese folklore, creating sparks by striking flint is considered to be lucky for someone leaving a house or group. The brief fire was once a way to drive evil spirits away. I found a reference to it being called kiribi, but there doesn't seem to be a whole lot of information on it.
Considering where Yaichiro is going as he leaves, it would be logical for a mother to wish her son good luck and to drive away evil from his path.
This ritual is not common anymore, but given the spiritual nature of Uchouten Kazoku, it makes perfect sense.