Opal, amorphous silica, and calcite occuring as veins, coatings on rocks or pebble, or as individual grains can be dated by U-Th or U-Pb isotopic methods using SHRIMP-RG. Both opal and calcite can be recrystallized at relatively low temperatures in the presence of fluids, so that U-Th or U-Pb ages can be complicated, and may reflect the minimum age for the sample.
The success of this approach relies on the uranium concentration of the samples. Kate Maher's group has found that a short-wave UV light can reveal the high-U regions of the silica. In general, >10 ppm U is needed for SHRIMP measurements.
The example above is from Oster et al. (2017). (A) Reflected light image of authigenic opal layers in soil (sample 22-3). Smooth, reflective-white continuous bands are opal. Brown discontinuous bands are calcite. The ages for individual SHRIMP-RG 230Th-238U spots are shown, and increase in age away from the edge (top). See paper for explaination of red box. (B) Same sample shown in a color cathodoluminescence image. Opal is bright green and displays banding corresponding to variations in U concentration. Calcite is non-luminescent. Epoxy is blue, and SHRIMP/RG spots are pink/red due to edge effects.
Maher et al., 2007, Quaternary International.
Maher et al., 2014, American Journal of Science.
Oster et al., 2017, Chemical Geology.