Is opalised shell a trace fossil?

No, an opalised shell is not a trace fossil. Trace fossils are evidence of biological activity or behavior, such as footprints or burrows, while opalisation refers to a process in which opal replaces the original material of a fossil, often resulting in a distinctively colorful appearance. Opalised shells are still considered body fossils, as the original shell material is preserved, albeit in a modified form.