Yeah, if rocks like that punctured my tires I'd be changing one flat per mile on my commute. I actually had better luck with the Kendra Kontenders (cheap training tires) than it sounds like you had with their Kwests - they gave me six months of flat free performance.

+1 on the Vittoria Randonneurs though, mine still look new after 5 months and ~ 2000 miles. And no flats.