"I am sick of being disbelieved," he said impatiently, and suddenly unbuttoning his wretched coat he pulled out a little canvas bag that was hanging by a cord round his neck. From this he produced a brown pebble. "I wonder if you know enough to know what that is?" He handed it to me. Now, a year or so ago, I had occupied my leisure in taking a London science degree, so that I have a smattering of physics and mineralogy. The thing was not unlike an uncut diamond of the darker sort, though far too large, being almost as big as the top of my thumb. I took it, and saw it had the form of a regular octahedron, with the curved faces peculiar to the most precious of minerals. I took out my penknife and tried to scratch it--vainly. Leaning forward towards the gas-lamp, I tried the thing on my watch-glass, and scored a white line across that with the greatest ease. I looked at my interlocutor with rising curiosity. "It certainly is rather like a diamond. But, if so, it is a Behemoth of diamonds. Where did you get it?" "I tell you I made it," he said. "Give it back to me." He replaced it hastily and buttoned his jacket. "I will sell it you for one hundred pounds," he suddenly whispered eagerly. With that my suspicions returned. The thing might, after all, be merely a lump of that almost equally hard substance, corundum, with an accidental resemblance in shape to the diamond. Or if it was a diamond, how came he by it, and why should he offer it at a hundred pounds? We looked into one another's eyes. He seemed eager, but honestly eager. At that moment I believed it was a diamond he was trying to sell. Yet I am a poor man, a hundred pounds would leave a visible gap in my fortunes and no sane man would buy a diamond by gaslight from a ragged tramp on his personal warranty only. Still, a diamond that size conjured up a vision of many thousands of pounds. Then, thought I, such a stone could scarcely exist without being mentioned in every book on gems, and again I called to mind the stories of contraband and light-fingered Kaffirs at the Cape. I put the question of purchase on one side. "How did you get it?" said I. "I made it." I had heard something of Moissan, but I knew his artificial diamonds were very small. I shook my head. "You seem to know something of this kind of thing. I will tell you a little about myself. Perhaps then you may think better of the purchase." He turned round with his back to the river, and put his hands in his pockets. He sighed. "I know you will not believe me."