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http://en.wikipedia.org/wiki/Kryptonite
In Superman Returns, an additional piece of kryptonite is found in a rock fragment, once more in Addis Ababa. Lex Luthor steals it from a Metropolis museum and uses it in his quest to create a new kryptonite landmass. During the extraction process, the rock appears to hold a significant amount of green kryptonite. The scientific name for the rock was displayed on its case, 'Sodium lithium boron silicate hydroxide with fluorine'. As borosilicate glass is commonly crystalline and green-tinted, this could be a plausible human mis-identification of kryptonite; alternately, as no 'unknown' component is listed, one might assume this (sodium/lithium/borosilicate/fluorine) blend to be the actual composition of green kryptonite. Though more likely, the researchers who performed the analysis of the fragment did not perform a core sample test. They may have only chipped off the outer layer in order to test it.