The black sapphire in the image is known as the Black star of Queensland, where black sapphires can be found. It was found at Anakie, a sapphire mining district in central Queensland, Australia.
The gemstone was found by the son of a miner whose surname was Spencer. The family do not, as far as I know, have any connection to the family of Princess Diana. For years the miner thought the stone was valueless; only later was its true value discovered, and that it was a star sapphire.