Complex sentence: The varnish is formed from the oxidization, or rusting, of the manganese and iron when mixed with oxygen in the air and water from rain; this varnish is dark, almost-black and glossy in appearance.
Split into simpler sentences: The varnish is formed from the oxidization or rusting of the manganese and iron when mixed with oxygen in the air and water from rain. This varnish is dark, almost-black and glossy in appearance.