Gold is the most non-reactive of all metals and is benign in all natural and industrial environments. Gold never reacts with oxygen (one of the most active elements), which means it will not rust or tarnish. Gold tarnish is very thin and shows up as a darkening of reflecting surfaces.
Gold is among the most electrically conductive of all metals. Since electricity is basically the flow of charged particles in a current, metals that are conductive allow this current to flow unimpeded. Gold is able to convey even a tiny electrical current in temperatures varying from -55° to +200° centigrade. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.
"Purple plague" is a brittle gold aluminum compound formed when bonding gold to aluminum. The growth of such a compound can cause failure in microelectronic interconnection bonds.
Another possible mechanism may be surface micro-porosity on the surface of investment (lost wax) cast items. This porosity may trap acids and other cleaning solutions, sprays, or perspiration and cause a local corrosion which 'creeps' over the surface of the item.
The tarnish films formed are generally harmless although unsightly and may lead to a black smudging of the skin. Such films can be easily polished off by a jeweler to restore the bright gold color.