The opal samples were cleaned after heavy liquid separation using a series of thorough reductive and oxidative cleaning steps (Shemesh et al., 1988; Ellwood & Hunter, 1999; Hendry and Rickaby, 2008) : 1) heating for one hour at ~80øC in 1% hydroxylamine chloride in acetic acid (1 g hydroxylamine chloride (AnalaR) dissolved in 10 ml glacial acetic acid (AnalaR) made up to 100 ml with 18 M-ohm.cm Milli-Q water), to remove surface oxides; 2) rinsing 3 times in Milli-Q; 3) heating for one hour at ~80øC in an etching reagent of 0.1% w/w sodium fluoride (AnalaR) solution to activate the opal surface; 4) rinsing 3 times in Milli-Q; 5) heating for two hours in a strong-acid solution of (AnalaR) quartz-distilled 16 N HNO3 (50%), 11 N HCl (10%) and Milli-Q water (40%), which acts as an oxidising agent to remove any remaining adsorbed particles and organics; 6) rinsing 5 times in Milli-Q.