1. Schrämli, W., and Becker, F., Ber. deutsch. keram. Ges., 37, 227 (1960).
2. Flörke, O. W., Ber. deutsch. keram. Ges., 32, 369 (1955).
3. An extensive bibliography on opal is given in Dana's The System of Mineralogy, 3, by Frondel, C. (John Wiley and Sons, Inc., New York, 1962).