Aha! Found it.

So at the National Museum in Edinburgh, I kept seeing the same symbol over and over and over again in Scottish jewelry: a circle with a line through it. Sometimes it was a super simple circle with a line across the middle, other times it was an elaborate Celtic torc that looked like it had been run through with a spike, but it showed up in every era of jewelry from Roman-era Picts all the way up to Victorian times. I was curious what it was, but all the plaques just called it a 'brooch,' which didn't really help.

Like these.

I eventually gave up and filed it away for looking it up later. Today I stumbled across the answer in, of all places, a Disney movie. I'm taking a pair of highland excursions in the next couple of days and to get in the right mindset for it, put on Pixar's Brave in the background while writing a post. I looked up and suddenly got my answer:

They used it to pin together their kilts!

I was very pleased to figure this out :)