Jade, the toughest gem material known to mankind, occurs as Nephrite & Jadeite. I should know, I break rocks for a living (no, not a prisoner): I'm a geologist and martial artist.
Jade is so tough that a rounded cobble or boulder of jade will resist breaking no matter how many times you strike it with a sledge. And be careful, there are a lot of jade look-a-likes sold by rock shops that is nothing more than serpentinite - a soft mineral that is weakly magnetic.
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