Yeah, the only justification for 'toughness' was the Young's modulus, which does not tell you how resistant it is to breaking at all. And by that measure, sapph

Yeah, the only justification for 'toughness' was the Young's modulus, which does not tell you how resistant it is to breaking at all. And by that measure, sapphire [1] is 2.5 times tougher than steel [2]. [1] E = 7.4e7 psi (http://rubicontechnology.com/company/sapphire http://rubicontechnology.com/company/sapphire) [2] E = 2.9e7 psi (https://en.wikipedia.org/wiki/Young%27s_modulus https://en.wikipedia.org/wiki/Young%27s_modulus)