Are you planning on giving your girl friend a ring with a stone set in base metal? As far as my experience in matters of bling goes, white gold, yellow gold and platinum do not set off the buzzers. But if you're slipping her a stainless steel cubic zircona then just develop the need for a lavatory visit land side of security, slip her a £20 note and tell her you'll see her at duty free, darling. Thus you go through separately. Blinded by the scent of perfumes extraordinary she will fall for the ruse especially if you've been moaning about your wibbly wobbly gut on the way to the airport.