Transparent gemstones and the most recent supercontinent cycle

Certain hard and normally opaque minerals, both rare and common, occasionally crystallize as transparent or partly transparent ‘gem’ crystals. Such stones, designated ‘crystalline coloured gemstones’ (CCGs), form under highly constrained just-so ‘Goldilocks’ conditions that did not exist prior to the last supercontinent cycle. One set of gem-forming conditions arises during continent-to-continent collisions. Another comes into existence during the breakup of continents and produces distinctively different gems. Over 50 different CCG-forming metamorphic and pegmatitic minerals of the first type crystallize in certain orogenic areas near the edges of continents. In East Africa, the Himalayas, Pamirs and Hindu Kush, groups of CCG deposits formed near the leading edge of the upper plate during the collisions, and are associated with large scale circular arcs. Gems of the second type, which are for the most part blue–green–yellow magmatic sapphires, occur in conjunction with continental volcanism in eastern Australia, far eastern Russia, eastern China, Laos, Vietnam, Thailand, Rwanda, Cameroon, Nigeria and elsewhere. Orogeny causes deep fractures to be regenerated upward into younger rocks. CCG deposits of the first type are systematically associated with ancient fractures originating from below, vestiges of the Earth’s early history. Fractures may be older than the rocks in which they are observed.