It's a 63mm gold-plated spike with a gold nugget on the end of it. Judging by the way the spike extends slightly beyond the nugget, I'd guess it was intended for some kind of slot like a buttonhole — you couldn't put it in a tie. The long part of the pin seems too long for that, though (yet not long enough to be a hat pin).
This one comes in its own box, marked William Grant, Jeweller, Spurriergate, York, established 1931. The pin is therefore either younger than 1931 or in some other pin's box.
It's quite an unusual piece, at least insofar as I've never seen anyone wandering around with a gold nugget sitting in their buttonhole.
The nugget only weighs 4g, by the way, so don't figure on retiring on the proceeds if you break into my house and steal it...