Evidence of a comet that struck the Earth about 28 million years ago in what is now Egypt has been found in a shiny black pebble, according to a study published in Earth and Planetary Science Letters. Scientists analyzed the pebble, found years ago by an Egyptian geologist within silica glass, and determined the pebble was a remnant of a comet, not a meteorite. "Comets always visit our skies -- they're these dirty snowballs of ice mixed with dust -- but never before in history has material from a comet ever been found on Earth," said David Block of Wits University in Johannesburg, South Africa.

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