Wedding ring
From Tardis Wiki, the free Doctor Who reference
A wedding ring was worn by those who were married. (TV: The Wedding of Sarah Jane Smith)
The Twelfth Doctor wore one to commemorate his wedding with River Song. (TV: The Wedding of River Song, The Husbands of River Song, Extremis, Twice Upon a Time) Martha Jones did the same when she was engaged to Thomas Milligan, (TV: The Sontaran Stratagem) but later discarded it when she married Mickey Smith. (TV: The End of Time)
