Gemmological Properties of Opals from Spencer, Idaho, USA

Fifteen opal samples of gem quality from the Spencer Opal Mines in Idaho (USA) were examined. These opals display play-of-colour, with three of them having a pink body colour. The analysis was conducted using standard gemmological tools, along with non-destructive spectroscopic, chemical, and imaging methods. All samples were identified as opal-A, with their play-of-colour resulting from a regular stacking of hydrated silica spheres. However, at times, the spheres are large to allow diffraction. Opal-A are only occasionally found in nature as gems. Inclusion observed in the studied samples included alunite (KAl3(SO4)2(OH)6), anhydrite (CaSO4), and iron-rich spherules (hematite). The pink body colour in the studied specimens is caused by the scattering of light from reddish dendrites. The samples show significant stratification, with fluorescence ranging from green to blue to orange, the latter of which represents a new phenomenon in opals not associated with quinones. Additionally, the orange fluorescence is accompanied by emissions from Sm2+ in anhydrite inclusions.