Peter took the diamond out of his pocket again and set it precisely in the centre of Garth Steinman’s closed diary. Garth Steinman kept his eyes on Peter for more than ten seconds before he turned his attention to the stone.

‘Well, then,’ he said. On the other side of the room, the pretty blonde girl had sat up and taken notice.

While Peter and Samuel stood and watched him in silence, Garth Steinman laid the diamond on a sheet of white paper, and examined it closely. His cigarette now burned unnoticed in the ashtray, and the only sound in the room, apart from the faint splashing of the fountain in the hallway, was Steinmen’s hoarse breathing, in and out of his open mouth.

Now Steinman took out a 20 × loupe, and scrutinised the diamond for brilliance and fire, and also to check whether it contained any ‘clouds’, which are small clusters of bubbles; and ‘butterflies’, which are cleavage cracks; or ‘naats’, which are the knotty flaws that sometimes appear at the interface between twinned crystals.

After long minutes of study, he eventually put the diamond down again, and tucked his loupe into the breast pocket of his dressing-gown.

‘I am very seldom taken by surprise,’ he said, thickly. ‘I know the value and the whereabouts of almost every major gemstone in the world. I have friends in the Russian court; in Vienna; and in Berlin. I was a close friend of Cecil Rhodes before he died last year, and Mr Ernest Oppenheimer regularly sends me cables from De Beers, and dines with me whenever he comes to London, or to Berne.’

Garth Steinman paused, and then lifted up the diamond in his right hand. ‘This, however, has taken me by surprise. It is genuine, I can assure you of that. No paste diamond could be made to this size without its falsity being immediately obvious. And who would attempt to fake a stone of this immensity? It would be absurd. Mr Ransome — that is your name? — this diamond I believe to be the famous Rio Diamond, which was sold at auction in 1888 to a New York diamond dealer called Greenberg. I don’t remember how much it fetched, about two millions, I think. But its size, and its cut, and its colour — all correspond with what I know of the Rio.’

The blonde girl had now got up from the chaise-longue and crossed the room, the lacy train of her negligée dragging across the canary-yellow carpet. She stood a little way away from Garth Steinman’s desk, and stared at the diamond as if it were a hypnotic eye. Peter gave her a quick, friendly smile, but she did not return it.

‘How much do you think the diamond is worth now?’ asked Samuel.