A Snot Rock is a rock of mucus located in either the mouth or the throat. It is usually formed overnight when sitting mucus begins to harden. It is very disgusting and I myself have only witnessed it one time.
So I woke up this morning and as I'm about to get in the shower, I feel the need to cough. After I cough I feel a lump in my mouth and so I spit it out and a Snot Rock comes flying out.
1 more definition
This is where from being in a dusty area, you form small rocks in your nose from the collected dust. They look similar to popping rocks.
Bloke 1: Man this stupid dust in sydney, given me snot rocks.