Mass Effect & Mass Effect 3
Sytau is shrouded by a high-pressure atmosphere composed of carbon dioxide and sulphuric acids. The surface is composed of geologically young plains of volcanic basalts and rhyolites, with high concentrations of carbon and radioactives.
Sytau is rabidly volcanic; a dozen "supervolcanoes" 50-120 kilometers wide are erupting across the planet. Sytau's carbon content and constant volcanism make natural diamonds plentiful, if extremely costly and dangerous to recover.