The Opal Murders were committed by Crookshank and his helper, the mage Vrinday Kayu in 246 HE. Crookshank had several people digging in his houses for fire opals. As he didn't want others to learn of his opals, he had the diggers killed afterwards. There were three groups of diggers. The first with nine people and the second with eight. All of them were killed afterwards; the first group stabed with swords, the second killed with poison. The Provost's Guard, among them Rebakah Cooper came just in time to save the third crew of diggers from death, among them Jack Ashmiller.
The diggers were hired by one of Crookshank's associates. They were forbidden to tell someone else of their new job. Thus nobody knew where they went when the diggers vanished; their families believed they had just run off. The hired diggers were blindfolded and taken to the cellar of one of Crookshank's houses. There, they were told to dig. Once they entered the cellar they didn't leave it again. All they needed was brought to them and the diggers were watched by guards hired by Inman Poundridge. The guards had mage marks on them which would kill them if they told anyone of their work. When all the opals in one cellar were found the diggers were killed and buried in the cellar. Once they were dead another crew of diggers would be hired.
It wasn't easy for the Provost's Guard to find out about the dead diggers. The first to learn about it was Rebakah Cooper, who got the news from her dust spinners and the pigeons in the beginning of April of 246 HE. She told her Dogs and they promised her to look into the matter but they didn't have much hope as people vanished in the Lower City all the time. The fact that the diggers did as they were told and didn't spread the news of their new job made it even more difficult. The Dogs hadn't even heard a whisper about someone hiring diggers when the second crew died. They made it just in time to save the third crew.