We got a map for rock picking at a rock shop, and one of the locations they identified said we could find a variety of opal there, from potch (common opal) to the good stuff. We found a bunch of nice rock there, some of which appeared to be fragile stone in caliche (much of it opaque, some translucent). Thing is, I am not sure whether I am seeing actual caliche with opal/potch or some kind of crusted chalcedony. Can anyone tell me how to identify opal?