Can't find vault stone

I can't find vault stone even though I've been searching for hours. I was strip mining and I found several veins of chromatic iron, but not a single vault stone. I searched through many caves in the deep slate area, and still nothing. Finally, out of desperation I turned on spectator mode and flew through the deep slate caves for 30 minutes exploring all caves in miles radius and I still haven't seen a single one. My gamma is all the way up so it would be pretty easy to spot. What's the deal?