Onyx
From Tardis Wiki, the free Doctor Who reference
Onyx was a shade of black, which shared its name with a gemstone.
Captain Latimer wore an onyx black cravat in in 1892. (TV: The Snowmen)
John Lumic wore an onyx black cravat up to when he was converted into a Controller of the Cybus Cybermen. (TV: Rise of the Cybermen / The Age of Steel)