The British Museum - Rosetta Stone

The Rosetta Stone is a dark granite stone (often incorrectly identified as "basalt") which provided modern researchers with translations of ancient text in Egyptian demotic script, Greek, and Egyptian hieroglyphs. Because Greek was well known, the stone was the key to deciphering the hieroglyphs in 1822 by Jean-François Champollion, and in 1823 by Thomas Young. The discovery facilitated translation of other hieroglyphic texts.