Opal from selected localities.
USA | |
| Lindgren, W. and Ransome, F.L. (1906) Geology and gold deposits of the Cripple Creek district, Colorado. United States Geological Survey Professional Paper 54: 372-373. |
Rob Lavinsky (ex. Rochard Kosnar specimen) | |
Minerals of Colorado (1997) Eckel, E. B. |