A piece of obsidian.
obsidian definition by Webster's New World
Origin: Modern Latin obsidianus ; from Classical Latin Obsidianus (lapis), a faulty reading in Pliny (altered by associated, association with Classical Latin obsidium, a siege ; from obsidere: see obsess) for Obsianus (lapis), stone of Obsius, finder of a similar stone in Ethiopia
obsidian definition by American Heritage Dictionary
Origin: Latin obsidiānus, misreading of obsiānus (lapis), Obsian (stone), obsidian, after Obsius, a Roman who supposedly discovered it or a similar mineral.
obsidian - Science Definition