Since the hardness of jade (nephrite 6.5, jadeite 7) lies far from that of the diamond, it is astonishing to learn that this stone is the hardiest of all gemstones. Due to its unique matted structure - its felted fibers - it is a difficult object to break. Nephrite has greater compressive strength than steel, and that gives a good reason why there was so much interest in an anvil that was made of jade - and apparently used for more than an ornament.
Gems for the Taking by Mary L.T. Brown, 1971.