To "play pearl necklace" is to turn your perfectly innocuous conversation into something that could sound incredibly filthy, just at the time when someone comes into earshot, then observing their reaction and deriving much hilarity thereof. Derived from the original and most popular catchphrase to use when playing.
*waiting for the bus*
CheShA:"...So you comin out tonight? I fancy getting cheggers"
Tido:"Yeah, I reckon. Where you fancy going?"
*bus door opens*
CheShA: "...So then I gave her a pearl necklace.... 2 to town please Mr Bus driver"
Bus Driver: "....!!"