It seems strange that they're so sure it's from the seventh century given the Roman coins on it. Maybe they're what help date it? I'd be interested to know who's depicted on them, what they say and what language is used.
I would guess the coins are much older than the necklace -- the alternative would be that they're from the Byzantine empire -- and the necklace is being dated based on the grave in which it was found.
The alternative to the coins being Roman is that they are Roman?