For ten points, name this allotrope of carbon, the hardest known mineral.
Associated with eclogitic or harzburgitic xenoliths, this mineral is formed deep in the mantle and brought suddenly to the surface in pipes of lamproite or kimberlite.  Its structure, with atoms at tetrahedral corners, means its heat conduction is the highest known, as is its compressional resistance.  For ten points, name this allotrope of carbon, the hardest known mineral.
Associated with eclogitic or harzburgitic xenoliths, this mineral is formed deep in the mantle and brought suddenly to the surface in pipes of lamproite or kimberlite.  
diamond