One type of these substances forms in rock consisting of olivine and pyroxene, a basic rock known as peridotite. In addition to the harzburgitic variety, an eclogitic variety forms from the organic detritus pushed down by subduction. They are usually found in a blue igneous rock which forms at pipes and is called kimberlite, and calcite crystals mistaken for these substances were found in a volcanic tuff cone in Hawaii, a namesake "Head." For 10 points, name this allotrope of carbon, the hardest known naturally- occurring mineral.
One type of these substances forms in rock consisting of olivine and pyroxene, a basic rock known as peridotite. In addition to the harzburgitic variety, an eclogitic variety forms from the organic detritus pushed down by subduction. They are usually found in a blue igneous rock which forms at pipes and is called kimberlite, and calcite crystals mistaken for these substances were found in a volcanic tuff cone in Hawaii, a namesake "Head." For 10 points, name this allotrope of carbon, the hardest known naturally- occurring mineral.
One type of these substances forms in rock consisting of olivine and pyroxene, a basic rock known as peridotite.