Clary Runs on Dunkin'
When the going gets rough, the tough get coffee. At the hospital where Clary's mom is in a coma, Luke and Clary seek strength in the brew, despite how bad it may be. Luke makes more quality java at home, and often picks up a cup on the go when carting Clary around the city. This symbol doesn't point at larger issues within the novel (coffee doesn't equal innocence or anything fancypants like that); coffee's role in City of Ashes is to remind us that everything still takes place in good ol' New York.
They don't call it the City That Never Sleeps for no reason—everyone's hopped up on caffeine. Hmm, maybe instead of a Fearless Rune, Clary should have invented some sort of Keurig Rune. As we're told at one point:
As long as there was coffee in the world, how bad could things be? (3.14.8)
Um. This is actually totally true. Scratch everything we've said against her; Clary is wise. She's a dang modern-day Socrates.