Dark was falling from a dull and humid sky, and the lamps were beginning to struggle for brightness in Piccadilly, when the opal of Carmalovitch was first put into my hand. The day had been a sorry one for business: no light, no sun, no stay of the downpour of penetrating mist which had been swept through the city by the driving south wind from the late dawn to the mock of sunset. I had sat in my private office for six long hours, and had not seen a customer. The umbrella-bearing throng which trod the street before my window hurried quickly through the mud and the slush, as people who had no leisure even to gaze upon precious stones they could not buy...