Everything Under the Moon

What's a stone that signifies lightning with the intent of using it as a "focus" for spellcasting? That will be blessed in nature?

Fulgurites - lightening struck sand/silica/Quartz becomes glass called Lechatelierite.

Thank you

I ended up not needing it the forest led me to a tree that looks like it'd been struck and burnt by lightning with a kind of burnt dead piece sticking out that was ok to take.

Ooo, those are the best gifts