Period Jewellery and Circa Dating Antique Jewellery

When appraising antique jewellery it is important that the appraiser circa date the piece.  Circa dating means that the appraiser estimates the date that the piece was produced.   The appraiser can then identify the piece as belonging to a specific period.  How does an appraiser know this?

Antique Brooch Flickr Photo by Kotomicreations