Perhaps you've seen Pathetic Peripatetic's concept design for a sapphire and diamond TARDIS ring. Now that ring is a reality, just the thing for when you've your perfect companion.
Pathetic Peripatetic makes the ring in a diamonds-only design, but the sapphires with a single diamond on each side really evoke the image of a blue police box. He doesn't say how much the rings cost, but you can email GinoArizmendi@artgemsjewelers.com for pricing information and availability. I can't imagine the rings are inexpensive, but they may be just the thing for a true blue Whovian couple — or anyone who wants to carry a little Doctor Who with them all the time.