Onyx (colour)

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)