In the early Middle Ages the diamond was a rare, highly valued object, which was worn not as a decoration or as an object of beauty, but as a magical amulet. Indeed, since only natural unpolished stones were at first available and as these only occasionally have an attractive shape (and this only as a rule when they are small), it was not the appearance but other properties which gave the diamond its special position. As a decorative gem it was not at first rated as highly as the ruby or the pearl. Its real magical importance was due to its great hardness. Because of this hardness, by the simple and familiar process of sympathetic magic, it was firmly believed that diamond could endow its wearer with corresponding hardihood and manhood. . . . Diamonds were, therefore, often worn by knights and leaders on the battlefield.

—S. Tolansky, from The History and Use of Diamond, 1962.