Carbonado compared to carbon‑12, relative to typical diamonds). Carbonado exhibits strong luminescence (photoluminescence and cathodoluminescence) induced by nitrogen and by vacancies existing in the crystal lattice. Luminescence halos are present around radioactive inclusions, and it is suggested that the radiation damage occurred after formation of the carbonados, an observation perhaps pertinent to the radiation hypothesis listed below. The origin of carbonado is controversial. Some proposed hypotheses are as follows: None of these hypotheses for carbonado formation had come into wide acceptance in the scientific literature by 2008. Supporters of an extraterrestrial origin of carbonados propose that their material source was a
What Is and What Should Never Be descending riff [of "What Is and What Should Never Be"] is amazing: It's like a bow is being drawn back, and then it releases. The rhythm of the vocals is almost like a rap. It's insane — one of their most psychedelic songs." This was also one of the first songs recorded by the band for which Robert Plant received writing credit. According to rock journalist Stephen Davis, the author of the Led Zeppelin biography "Hammer of the Gods: The Led Zeppelin Saga", the lyrics for this song reflect a romance Plant had with his wife's younger sister. "What Is