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. (DW: Tooth and Claw)

In 1909, while working for an unknown employer, Jack Harkness stole diamonds from a mine in Lahore. (WEB: torchwood.org.uk)

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. (SJA: 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 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. (DW: The End of Time)

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

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

The planet Midnight's surface was mostly diamonds, but since the planet went through harsh Xtonic radiation, the diamonds could not be excavated. (DW: 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. (DW: The Infinite Quest)

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

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