28 January 2000
Vol 287, Issue 5453

About The Cover

Cover image expansion

COVER An emerald in its gangue. This crystal (5 mm in length) is from the main Colombian mine of Coscuez. The oxygen isotopic ratio of Coscuez emeralds is diagnostic and can be used to trace the origin of historical gems, especially "old mine" emeralds that were cut in the 18th century in India. See page 631 [Photograph: Hervé Conge; reprinted with permission from Association Française de Gemmologie, CNRS, IRD]