Ashkenaz jewery is likely from Rome. We do know that there was a significant Jewish community in Rome at the time of the Mishna. It makes sense that many of them migrated to general Europe during the time of Charlemagne.
Fairly recently some group came across an old Jewish cemetery and tested their DNA form their teeth. It showed a very varied background, suggesting merchants who happened to grab the opportunity and came to Europe for business. This makes sense at that point since the aforementioned Charlemagne invited Jews and made things safe.