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; the Androvar was sent flying into his head, like shrapnel, as a result. 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 buyers were the Shoal of the Winter Harmony, who were enraged when they learned that the diamond was still inside Hydroflax's head, as they were loyal subjects of his. Later, Hydroflax inadvertently ordered his robot body to destroy his head, liberating the Androvar. When the Harmony and Redemption was crippled by a meteor storm shortly afterwards, 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) In accordance with the Doctor's instructions, the diamond was returned to the Halassi on 28 July 5343. (PROSE: Time Traveller's Diary)