Demon's / Caliph of the Sands/ mysterious man

He let out a chuckle, then said,  “You really are quite composed, to be able to wait so long before coming out.”

 “Nothing.” The black-garbed man withdrew his gaze, then said slowly, 

 “That stone is completely useless to you. If you are able to find it in the future, I’ll pay you a hundred times the commission you earned today!”

“Wait a moment!” For the first time, the black-garbed man spoke. His gravelly voice sounded like that of a tightly controlled wild beast’s, and when it came out from that breathing mask it sounded incredibly strange. 

 “Did any of you find anything unusual? For example, a strange rock.”