Oh, it's definitely true. When Brits riot, they throw "stones". Americans think the word reminds them of testicles and so throw "rocks" instead.

Which to Brits is ridiculous as to them "rocks" are rather larger bits of stone that are pretty hard to pick up, let alone throw.

I'm told. Not being a native speaker of either British or American English I wouldn't know if this is true or not, but I thought the story was amusing enough to share.