As Rompson wrote, more or less, it's a pearl tie pin - your classic Victorian/Edwardian accessorising at work. They're especially useful/traditional at weddings
I've a habit of buying things in pairs, so I'll found another one quite recently, which is set in solid gold
Many thanks to you and the others for your compliments
I never would of thought that those colours - namely the pants and the tie - would work with the rest of the outfit but they do very nicely.