Correction
OCT 01, 2006
DOI: 10.1063/1.2387075
June 2006, page 16 — The description of opal’s chemical composition is wrong. Opals contain very little calcium carbonate. Their chemical formula is SiO2·nH2O, where n is usually between 0.5 and 2. Thanks to Andrew Locock of the University of Alberta in Edmonton for pointing out the mistake.
© 2006 American Institute of Physics
