Halassi Androvar

The Halassi Androvar was the most valuable diamond in the universe in the 54th century. An ancient artefact of the Halassi people, it was kept inside the Halassi Vaults.

In 5343, King Hydroflax led a raid on the Vaults, which led to the Vaults exploding and sending the Androvar flying into his head. The Halassi tasked River Song with retrieving the diamond, though she actually intended to sell it herself.

Taking Hydroflax's living head into their possession, River and the Twelfth Doctor travelled to the Harmony and Redemption, where River had arranged to sell the Androvar for 100 billion credits. The buyer was the Shoal of the Winter Harmony, who were enraged when they learned where the diamond was as they were Hydroflax's subjects. Hydroflax's head inadvertently ordered his robot body to destroy him, liberating the Androvar. When the Harmony and Redemption was crippled by a meteor storm shortly after, River caught the Androvar in her bodice.

After the Harmony and Redemption crashed on Darillium, the Doctor gave the Androvar to Alphonse, telling him to return it to the Halassi and build a restaurant on that spot with the reward money. (TV: The Husbands of River Song)