ad·a·man·tine 
(ăd′ə-măntēn′, -tīn′, -tĭn)
(ăd′ə-măntēn′, -tīn′, -tĭn)adj.
1. Made of or resembling adamant.
2. Having the hardness or luster of a diamond.
3. Unyielding; inflexible: "If there is one dominant trait that emerges from this account, it is adamantine willpower" (Eugene Linden).