How do I stop weeds in desert landscaping?
It might be a bit late, as the rock's already down ... but the easy method would've been to put down a layer of landscape fabric before you put down the inch of rock.

I have no idea how much rock would stop grass ... I'd think there'd be other factors in play (eg, what size of rock, amount of void space, how good the soil is underneath, etc.)

If you don't have much grass nearby, and you didn't do this in an attempt to kill off an existing lawn, the seeds likely came in with the fill soil; if you pull the grass as it comes up (before it goes to seed), you should be able to kill it off