Cracking of Gem Opals

The value of gem opals is compromised by their potential susceptibility to crazing, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: water-stored opal samples, which are stored in water after extraction, and air-stored opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 degrees C, while for air-stored opals, higher temperatures, circa 300 degrees C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking.

Automated summary

The susceptibility to cracking in gem opals is affected by the method of storage, with water-stored opals requiring lower temperatures to induce cracking compared to air-stored opals. Cracking in water-stored opals is attributed to drying shrinkage, while cracking in air-stored opals is attributed to super-heated water-induced decrepitation.