Carbonado; diamond aggregates from early impacts of crustal rocks?

Carbonados are irregular polycrystalline aggregates of diamond that occur in placers and low-grade metamorphic rocks. They are characterized by fine grain size, light C isotope ratio, a wide variety of inclusions of crustal materials (silicates, phosphates and oxides) and an absence of minerals characteristically associated with diamond from the upper mantle. To explain these and other properties it is proposed that carbonados were formed from carbon-rich materials by large impacts on Earth's crust in the Precambrian era and survived subsequent alteration of the impact debris to become incorporated into sediments and low-grade metamorphic rocks.