I have only found one for myself, quite by accident, this black one, on the shores of Belfast Lough. Pocket sized.
Earlier this year, I was on the beautiful beach at Newcastle, County Down, and my (now taller-than-me son) decided he'd hunt for a heart. The stones and rocks there were blue, with fabulous stripes on them, but they were smooth and rounded and I reckoned he'd not succeed.
But he certainly did. Not only did he find THREE (a bumper crop!) heart shaped stones.
but he also noticed a really big heart. He was walking ahead of me and stopped in his tracks, and everything about his body language said - I don't believe it. He pointed and said - its there. LOOK!
The moral of this story? Be aware. Look. And laugh when everything works perfectly!