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Jade is between 5.5 to 6.5 on the mohs scale, but a hard stone doesn't necessarily means it will be more difficult to lap than a softer one. Marble (mohs hardness 2-3) is harder to lap than glass....

Jade hones are are not rare, and supposedly very fine, but their performance vary a lot, be that jadeite or nephrite. And, they are not expensive compared to any other natural hone. 15x5cm hones cost...