Diamond

You may be looking for Lady Diamond.

Diamonds were a form of crystallised carbon and one of the most valuable substances in the universe.

In 1879, the Tenth Doctor used the infamous Koh-i-Noor diamond to kill a werewolf by using it to reflect enough moonlight from the light chamber to be fatal to it. (TV: Tooth and Claw)

Around the turn of the 20th century, the Fourth Doctor used a diamond as a focusing piece to turn a lighthouse into a giant laser in order to destroy a Rutan warship. (TV: Horror of Fang Rock)

In 1909, while working for an unknown employer, Jack Harkness stole diamonds from a mine in Lahore. (TV: Small Worlds)

In 2009, members of the Slitheen family attempted to compress the planet Earth into a diamond, but were stopped by members of the Slitheen-Blathereen family. (TV: The Gift)

To escape the Last Great Time War, the Time Lords sent a White-Point Star in the form of a diamond through the time-lock to the Saxon Master. He used it to amplify the signal and bring them to Earth, but the Tenth Doctor shot it with Wilfred Mott's old service revolver, destroying it and the link. (TV: The End of Time)

When Vislor Turlough won Enlightenment, he at first believed it to be a huge diamond. (TV: Enlightenment)

Diamonds were common on Telos, where the Cryons offered to pay Lytton and Griffiths in uncut diamonds in return for their services. (TV: Attack of the Cybermen)

The planet Midnight's surface was mostly diamonds, but since the planet went through harsh X-tonic radiation, the diamonds could not be excavated. (TV: Midnight)

In the 40th century, Baltazar attempted to compress the planet Earth into a diamond, but was stopped by the Tenth Doctor and Martha Jones. (TV: The Infinite Quest)

The diamond mines of Barzilla were abandoned by the time Sara Kingdom visited that planet. (PROSE: The Outlaw Planet)

The planet Sartor was rich in diamonds, which were mined by the human colonists. Quarsian pirates attempted to steal the diamonds, but were defeated by the Tenth Doctor. (COMIC: The Diamonds of Sartor / Quarsian Mission / Android of Death)

In the distant future, one of the Three Great Treasures of Generios was the largest diamond in existence, which was guarded by a Jelloid. (AUDIO: The One Doctor)

At the end of the universe, it was believed that the skies of Utopia were made of diamonds. (TV: Utopia)

Diamond Spiders created giant diamond webs capable of holding asteroids in place. (PROSE: Web in Space!)

The star-liner Excelsis was powered by zero-point energy focused through the Eye of Ashaya, the largest diamond in the galaxy. (COMIC: The Eye of Ashaya)