1. School of Geographical and Earth Sciences Gregory Building, University of Glasgow Glasgow G12 8QQ, U.K.
2. Gemological Institute of America New York City, NY 10128, U.S.A.
3. Department of Earth and Atmospheric Sciences University of Alberta Edmonton T6G 2E3 Alberta, Canada
4. Department of Earth and Environmental Sciences, Dalhousie University, Halifax, Canada
5. Royal Holloway University of London, Egham, Surrey TW20 0EX, U.K.