Here they are talking about surface quenching, where the amorphous carbon thin film is heated then rapidly cooled on the surface. Instead of forming smaller precipitates, like in an alloy, it just forms very small crystals that happen to be extremely hard. They are embedded in amorphous material. The key phrase that capture this is: &quot;This Q-carbon has amorphous structure with some embedded nanocrystalline diamonds the number density of which is determined by the nucleation time available for growth.&quot; Hopefully this clears things up somewhat.<p>Pure carbon is actually metallic at sufficiently high temperatures, as reported in this paper and in this citation: [1].<p>1. F. P. Bundy , W. A. Bassett , M. S. Weathers , R. J. Hemley , H. U. Mao , and A. F. Goncharov , Carbon 34, 141 (1996).<a href="http:&#x2F;&#x2F;dx.doi.org&#x2F;10.1016&#x2F;0008-6223(96)00170-4" rel="nofollow">http:&#x2F;&#x2F;dx.doi.org&#x2F;10.1016&#x2F;0008-6223(96)00170-4</a>