Carbonado in the pores that are common in carbonados, they may have been introduced after carbonado formation. Inclusions of other minerals, rare or nearly absent in the Earth's crust, are found at least partly incorporated in diamond, not just in pores: among such other minerals are those with compositions of Si, SiC, and Fe‑Ni. No distinctive high-pressure minerals, including the hexagonal carbon polymorph, lonsdaleite, have been found as inclusions in carbonados, although such inclusions might be expected if carbonados formed by meteorite impact. Isotope studies have yielded further clues to carbonado origin. The carbon isotope value is very low (little carbon‑13
Rock Eagle Rock Eagle Rock Eagle Effigy Mound is an archaeological site in Putnam County, Georgia, U.S. estimated to have been constructed c. 1000 BC to AD 1000 (1,000 to 3,000 years ago). The earthwork was built up of thousands of pieces of quartzite laid in the mounded shape of a large bird (102 ft long from head to tail, and 120 ft wide from wing tip to wing tip). Although it is most often referred to as an eagle, scholars do not know exactly what type of bird the original builders intended to portray. It is listed on the National Register