Moulding et al. (2008): "Robert Philips Greg described "minute but measurable crystals, on macled crystals of black carbonate of lead from Force Craig lead mine near Keswick", and provided a chemical analysis and a crystal drawing to support his identification. Sir Arthur Russell (1944) cast doubt on the record following extensive research on the Allan-Greg collection, as it was not substantiated by either a specimen or a catalogue entry. His views are echoed by more recent workers (e.g. Cooper and Stanley, 1990)."
