Patina is “a thin layer that forms on the surface of stone; copper, bronze and similar metals (tarnish produced by oxidation or other chemical processes)”. It is also a great technique to give pennies a pretty colour!!
This tutorial by Buttons and Paint shows you how to use vinegar and salt to create a lovely patina finish on coins. It gives a range of pretty colors in blues, turquoises and golds. The finished result could be used in a variety of indie projects!