The extracting of a person's nasal mucus (UK term bogeys - referred as 'boogers' in the US) from one's nasal passages. The term 'mining for emeralds' came about due to the nasal materials' similarity in colour to the emerald jewel.
1. "Oops, nearly got caught mining for emeralds there!"
2. "Urgh, have you seen the bog - someone's been mining for emeralds all across the wall!"