The Veiled Leopard was a crystal which became famous on Earth as a precious diamond, but was in fact a storage device of alien origin.
The doomed population of a dying planet created the crystal and encoded the data to rebuild their entire civilisation into it, ensuring their world would live on. They launched the crystal into space aboard a ship, which was to take it to a new planet where their civilisation could be rebuilt from the data, but other aliens destroyed the ship in an attempt to secure the crystal's technology for themselves. The Doctor rescued the crystal from the fight. Since the patterns encoded in the crystal were too unstable to endure time travel, the Doctor took it to ancient Earth and made sure it found its way into royal hands, where it would be safeguarded until the time was right to take it to a new planet. Believing it to be a diamond of incredible value, the humans named the stone "the Veiled Leopard" due to a particular flaw in it which resembled a leopard's spots.
Pharaoh Amenhotep II of Egypt once claimed the stone as the spoils of war, and it became his prize possession. Amenhotep's daughter Erimem had the stone buried with her father in his pyramid when he died.
The stone was removed from Amenhotep's tomb by grave robbers, and it eventually found its way into the possession of industrialist Gavin Walker, who announced that he would give it to his wife at a party in Monte Carlo in 1966. Secretly, Walker was arranging for Bennett, a jewel thief under his own employ, to steal the diamond during the party so he could collect the insurance.
The time was right to send the crystal to its new planet so it could restore the civilisation of its creators, so the Fifth Doctor (acting reluctantly upon the instructions of his future self) sent Peri Brown and Erimem to the party to prevent the stone from being stolen by jewel thieves. The Seventh Doctor sent Ace and Hex to the same party, to steal the stone themselves.
While Peri and Erimem successfully prevented Lillian Hawthorne, a wealthy jewel thief who targeted the dishonest and gave the proceeds of her crime to charity, from stealing the Veiled Leopard, Ace and Hex cooperated with Lillian's accomplice Peter Mathis to steal it and substitute it with two forgeries; one for Mathis to sell on the black market as he would have the genuine one, and another to leave for Bennett to steal and get caught with, exposing Walker's insurance scam. After tripping up Bennett and exposing the forgery in front of the party's guests, Ace and Hex moved to leave with the Veiled Leopard. They were interrupted by Lillian, who demanded to know the whole story despite Ace's insistence that she'd never believe it. (AUDIO: The Veiled Leopard)