It Is The World's Biggest Black Sapphire,

but was once used as a doorstop! 

photograph via news.internetstones.com it is known as the Black Star of Queensland, the 733-carat stone was discovered in the 1930s in the Anakie Sapphire fields of the Rubyvale area in the State of Queensland, in north-eastern Australia, and yes it was used as a doorstop, weighing over a thousand carats it shed a bit of weight when it was cut, from the article:

"The enormous gemstone was discovered accidentally in 1938 by a young boy of around 12 years named Roy Spencer when he was playing around in the claim where his father used to prospect for sapphires, known as the Reward claim, which lies within the Anakie Sapphire fields in the Rubyvale area, in the State of Queensland. The Anakie Sapphire fields is the largest of all sapphire mining areas in Australia with an area of around 900 sq. km. Immediately after picking up the enormous black crystal the boy lost no time in running back home to show his accidental find, to his father Mr. Harry Spencer, who was one of the pioneering miners of the central Queensland gemfields. Mr. Harry Spencer was not impressed, and after examining the enormous black crystal just threw it aside, saying it was just a large black crystal. Perhaps it was not known at that time that sapphires could also exist as black gemstones. The enormous black crystal was then fortunately used as a door stop, in the Harry Spencer household for around a decade, without being thrown away as something worthless"

the story of the Black Star and what exactly a black sapphire is, and a nice read as it happens, can be found here.