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 a specialy constructed telescope to be fatal to it. (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. This would be another of their failed constant attempts to use the Earth for profit in some way.(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)

The planet Barzilla had abandoned diamond minde by the time Sara Kingdom visited it. (DAN: 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. (DWBIT: 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. (BFA: The One Doctor)

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

When arriving on a spaceship, the Eleventh Doctor, Amy Pond and Rory Williams realised that they were caught in a giant Diamond Spider's web, which was a rare species in the universe. It created diamond web structures, which were able to hold asteroids in their place. (NSA:  Web in Space!)