In his Naturalis Historiae, published in 77 AD, Pliny wrote about diamonds' usefulness as tools:
These stones are tested upon the anvil, and will resist the blow to such an extent, as to make the iron rebound and the very anvil split asunder. Indeed its hardness is beyond all expression ... When, by good fortune, this stone does happen to be broken, it divides into fragments so minute as to be almost imperceptible. These particles are held in great request by engravers, who enclose them in iron, and are enabled thereby, with the greatest facility, to cut the very hardest substances known.