That did it I think. I must have ridden over some glass earlier in the day and rolling over that rock drove it home. At least i think it was glass. Otherwise there was a hard sharp brown shard of something on my shirt with no idea how it got there. I can't feel anything in there now and it took me two passes around the tire with my thumb rubbing it to find the hole again.

I'll try the superglue trick. Should I put any on the inside of the tire as well? The cut did come all the way through.