Updated as of April 1st, 2011
Updated as of July 27th, 2012
Hearthstones:Gem of Adamant Skin
From Exalted Wiki
Gem of Adamant Skinhearthstone is a glittering, flawless black ovoid. The bearer becomes impossible to cut, and her bones become unbreakable.
Source: Second Edition Core, p. 382
Source: First Edition Core, p. 339