Gemstones were pieces of mineral crystal.

At the end of the 20th century, only four gemstone were still in existence, namely the Hazandra, the Arquess, the Alcyone and the Sanguinare. (COMIC: Ghost Stories)

The Twelfth Doctor once owned the hazandra gemstone that Grant ingested. (TV: The Return of Doctor Mysterio) The result was Grant gaining "superpowers" that he used as his alter ego "the Ghost". (TV: The Return of Doctor Mysterio, COMIC: Ghost Stories)