Diamond

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

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

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)

In the 40th century, Baltazar attempted to compress the planet Earth into a diamond, but was stopped by the 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)