I was really hoping the archaeologists were trying to fob us off with a buckle instead of admitting it's a piece of extra-terrestrial technology!
Now you mention it, that does seem the only plausible explanation for a polished rectangle of rock with some coloured stones set into it, placed on the waist.
Why is everything rectangular now described as looking just like an Iphone? From the comments below the article:
I just put a paving slab on my living room wall, no different to a 4k UHD TV. The picture is great.